Internal ballistics
|
|
Internal ballistics is the science of the process of firing a firearm from the process of igniting the propellant to the exit of the projectile from the barrel. See also Transitional ballistics, External ballistics, and Terminal ballistics. The study of internal ballistics is important to designers and users of firearms of all types, from smallbore Olympic rifles and pistols, to high-tech artillery.
Ignition
Priming methods
The first step to firing a firearm of any sort is igniting the propellant. The earliest firearms were cannons, which were a simple closed tube. There was a small hole, the touchhole, drilled in the closed end of the tube, leading down to the main powder charge. This hole was filled with finely ground powder, which was then ignited with a hot ember or torch. With the advent of hand-held firearms, this became an undesirable way of firing the gun. Holding a burning stick while trying to carefully pour a charge of black powder down a barrel is a good way to get maimed or killed. Also, trying to hold the gun with one hand, aim at the target, and look for the touchhole so you could put the burning stick against it with the other hand wasn't conducive to any degree of accuracy.
External priming--match, wheel, flint, cap, modern muzzleloaders
The first attempt to make the process of firing a small arm easier was the matchlock. The matchlock incorporated a "lock" (so called because of its resemblance to door locks of the day) that was actuated by a trigger. The lock was a simple lever which pivoted when pulled, and lowered the match down to the touchhole. The match was a slow burning fuse made of plant fibers that were soaked in a solution of nitrates, charcoal and sulphur and dried. This was ignited before the gun might be needed, and it would slowly burn, keeping a hot ember at the burning end. After the gun was loaded and the touchhole primed with powder, the burning tip of the match was positioned so that the lock would bring it into contact with the touchhole. To fire the gun, it was aimed, and the trigger pulled. This brought the match down to the touchhole, igniting the powder. The slow burning match could be kept going, with careful attention, for long periods of time, and the use of the lock mechanism made accurate fire (within the limits of the gun) possible.
The next revolution in ignition technology was the wheel-lock. It used a spring loaded, serrated steel wheel which rubbed against a piece of iron pyrite. There was a key which was used to wind the wheel and put the spring under tension. Once tensioned, the wheel was held in place by a trigger. When the trigger was pulled, the serrated edge of the steel rubbed against the pyrite, generating sparks. These sparks were directed into a pan, called the flashpan, filled with loose powder which lead into the touchhole. The flashpan was usually covered by a spring loaded cover that would slide out of the way when the trigger was pulled, exposing the powder to the sparks. The wheellock was a major innovation--since it did not rely on burning material as a source of heat, it could be loaded and kept loaded for extended periods of time. The covered flashpan also gave the gun some ability to withstand bad weather. Wind, rain, and wet weather would render a matchlock useless, but a wheellock that was loaded and waterproofed with a bit of grease around the flashpan could be fired under most conditions.
The wheellock enjoyed only a brief period of popularity before being superseded by a simpler, more robust design. The flintlock, like the wheellock, used a flashpan and a spark to ignite the powder. As the name implies, the flintlock used flint rather than iron pyrite. The flint was held in a spring loaded arm called the "cock", after its resemblance to a rooster. The cock rotated through about a 90 degree arc, and was held in the tensioned or "cocked" position by a trigger. Usually, flintlocks would lock the cock in two positions. The "half cock" position held the cock halfway back, and used a deep notch, so that pulling the trigger would not release the cock. This was a safety position, used when loading and when storing or carrying a loaded flintlock. The "full cock" position held the cock all the way back, and was the position from which the gun was fired. The frizzen was the other half of the flintlock ignition system. It served as both a flashpan cover and a steel striking surface for the flint. The frizzen was hinged, and spring loaded so that it would lock in the open or closed position. When closed, the striking surface was positioned so that the flint would strike at the proper angle to generate a spark. The striking flint would also open the frizzen, exposing the flashpan to the spark. The flintlock mechanism was simpler and stronger than the wheellock, and the flint and steel provided a good, reliable source of ignition. The flintlock remained in military service for over 200 years, and flintlocks are still made today for historical reenactments and hunters who enjoy the additional challenge the flintlock provides.
The next major leap in ignition technology was the invention of the chemical primer, or "cap", and the mechanism which used it, called the "caplock". The caplock appeared just before the American Civil War, and was quickly adopted by both sides as it was even simpler and more reliable than the flintlock. The main reason the caplock was so quickly adopted was its similarity to the flintlock. The flashpan and frizzen were removed, and replaced by a "nipple" which the cap fit onto. The cock was replaced by a "hammer", which also had half cock and full cock positions for the same reasons. When fired, the hammer would hit the cap, crushing it onto the nipple. The cap was a thin metal cup that had in it a small quantity of pressure sensitive explosive. When crushed, the explosive would detonate, sending a stream of hot gas down a hole in the nipple, and into the touchhole of the gun. In the process of firing, the cap generally split open, and would fall off when the hammer was moved to half cock position for loading. The caplock system worked well, and is still the preferred method of ignition for hunters and recreational shooters who use muzzleloading arms.
Internal priming--Berdan, boxer, rimfire
Chemical primers, advanced metallurgy and manufacturing techniques all came together in the 1800s to create an entirely new class of firearm--the cartridge arm. Flintlock and caplock shooters had long carried their ammunition in paper cartridges, which served to hold a measured charge of powder and a bullet in one convenient package (the paper also served to seal the bullet in the bore). Still, the source of ignition was separate. With the advent of chemical primers, it wasn't long before all sorts of systems were invented, with a multitude of different ways of combining bullet, powder, and primer into one package which could quickly be loaded from the breach of the firearm. The three systems which have survived the test of time are the rimfire, the Berdan primer, and the boxer primer.
Rimfire cartridges use a thin brass case with a bulge, or rim, around the back end. This rim is filled during manufacture with an impact sensitive explosive. When the rim is crushed by the hammer or firing pin, the explosive detonates and ignites the powder charge. Rimfire cartridges are single use--after firing, they cannot practically be reloaded. Also, since the rim must be thin enough to be easily crushed, the pressures generated in the case are limited by the strength of this thin rim. Rimfire cartridges used to be available in calibers up to .44, but all but the small .22 caliber rounds died out. The .22 Long Rifle (which is also fired in pistols) is the most popular recreational caliber, because it is inexpensive, quiet, and has very low recoil. The most inexpensive brands can be bought for less than US$0.02 per round in boxes of 500, and even Olympic class ammunition is around US$0.20 per round.
The remaining types of priming, Berdan and Boxer are both considered "centerfire" to differentiate them from the rimfire rounds. Centerfire priming methods are interchangeable--the same firearm can fire both Berdan and Boxer primed rounds. Berdan primers are named after their American inventor, Hiram Berdan. These are very similar to the caps used in the caplock system--they are small metal cups with pressure sensitive explosive in them. The Berdan primers are pressed into the primer "pocket" of a Berdan type cartridge case, where the fit flush with the base of the case. Inside the primer pocket is a small bump, the "anvil" that rests against the center of the cup, and two small holes that allow flash from the primer to reach the interior of the case. Berdan cases are reusable, although the process is rather involved. The used primer must be removed, usually be hydraulic pressure or a lever that pulls the primer out the bottom. A new primer is carefully seated against the anvil, and then gunpowder and a bullet are added. Berdan priming is used by nearly all militaries and most civilian manufacturers, with the ironic exception of the USA.
Boxer primers, named after their inventor, British military officer Edward M. Boxer, are similar to Berdan primers with one major change--the anvil. In a Boxer primer, the anvil is a separate piece that sits in the primer cup. Because of this, the primer pocket has the flash hole centered. This makes little or no difference in the performance of the round, but it makes fired primers vastly easier to remove. A thin metal rod is pushed through the mouth of the case, and it pushes the primer out. A new primer, anvil included, is then pressed into the case. Since the primer and anvil are sold as one part, the anvil depth is correct for the primer that is being inserted, so that the primer does not ignite during loading (although priming is done as the first step, before the powder is added, just in case). This is the main reason that Boxer priming is still popular in the USA, as there are a large number of shooters who reload their ammunition. Boxer primed ammunition is slightly more complex to manufacture, since the primer is in 2 parts, but the slight increase in initial cost is more than made up for by the decreased cost of firing reloaded rounds.
Electrical
Lock time considerations
Nearly all firearms today still use essentially the same technology that was developed for the caplock--a spring loaded hammer or striker is used to crush the primer, which causes a small detonation that ignites the powder charge. Since this system is mechanical, there is a small but significant lag, called the "lock time", involved between the pull of the trigger and the ignition of the cartridge. This is usually only a few hundredths of a second, but when you consider that the bullet leaves the barrel in a few thousandths of a second after ignition, the lock time is the major source of delay. When the shooter is trying to time a shot to fire between heartbeats, those hundredths of a second can mean the difference between a gold medal and no medal at all. Another place where lock time is critical is in the multibarrel Gatling guns used by militaries, which have rates of fire as high as 100 rounds per second. Many high speed Gatling guns, and a growing number of civilian arms are switching to electrical primers. [Grammatical notice] Electrical primers in civilian arms use a battery pack and a capacitor to build up an electrical charge that is then sent through a transformer to increase the voltage to 150 to 200 volts. This charge is then sent through the primer where it ignites the explosive and thus sets off the propellant charge. Larger bore military weapons use thousands of volts to create a spark which detonates the primer.
Propellant
Black powder--constant burn rate, not very efficient, very dirty
Black powder was the first chemical propellant and the first explosive recorded by history. Black powder is a mix of sulphur, charcoal, and potassium or sodium nitrate. It acts more like an explosive than a propellant, since its burn rate is not affected by pressure, but it is a very poor explosive because it has a very slow decomposition rate and therefore a very low brisance. This same property that makes it a poor explosive makes it useful as a propellant--the lack of brisance keeps the black powder from shattering the barrel, and directs the energy to propelling the bullet. The main disadvantages of black powder are a relatively low energy density (compared to modern smokeless powders) and the extremely large quantities of soot left behind. During the combustion process, less than half of black powder is converted to gas. The rest ends up as a thick layer of soot inside the barrel and a dense cloud of white smoke. In addition to being a nuisance, the residue in the barrel is hydrophilic and an anhydrous caustic substance. When moisture from the air is absorbed, the potassium or sodium oxide turn into hydroxides, which will eat away the barrel. Black powder arms must be well cleaned inside and out after firing to remove the residue. Black powder differs from the smokeless propellants in that its burn rate is not significantly affected by pressure. The size of the granules of powder and the confinement determine the burn rate of black powder. Finer grains result in a closer mix of the ingredients, which results in a faster burn. Tight confinement in the barrel causes a column of black powder to burn from end to end, which is the desired method. Not seating the bullet firmly against the powder column can result in the loose powder burning all at once, which can create a dangerous overpressure condition. It is difficult to overload a black powder arm, as any excess powder will simply be blown unburned from the end of the barrel. In fact, that is a traditional method for determining the optimal loading for a black powder musket or rifle--fire it over a patch of virgin snow with gradually increasing powder charges, and look for the appearance of unburned powder grains in the snow to indicate too much powder. The lack of pressure sensitivity means that the mass of the bullet makes little or no difference to the amount of powder used. A full charge of black powder seated by just a small wad of paper, with no bullet, will still burn just as quickly as if it had a full weight bullet in front of it. This makes black powder very well suited for blank rounds, signal flare rounds, and rescue line launching rounds.
Nitrocellulose--burn rate proportional to pressure, clean, high density
Nitrocellulose is formed by the action of nitric acid on cellulose fibers. Nitrocellulose is a highly combustible plastic that deflagrates rapidly when heat is applied. It burns very cleanly, burning almost entirely to gaseous components at high temperatures. The burning rate of nitrocellulose is dependent upon the pressure--a pile of uncontained nitrocellulose will burn slowly, with a high, bright flame, but when placed in a high strength sealed container, the same material will burn very quickly, bursting the container. Since nitrocellulose is a plastic, it can be formed into many shapes of gunpowder, such as cylinders, tubes, balls, and flakes. The size and shape of the powder grains can increase or decrease the relative surface area and change the burn rate significantly. Additives and coatings can be added to the powder to further modify the burn rate. Very fast powders are used for low velocity pistols and shotguns, medium rate powders are used for magnum pistols and light rifle rounds, and slow powders are used for large bore heavy rifle rounds.
Double base propellants--nitrocellulose plus nitroglycerine, high energy fast burn
To further increase the energy of smokeless powder, nitroglycerin can be added in amounts up to 50%. These powders are called double base powders, and they have the same basic physical properties as single base powders. The nitrocellulose serves to desensitize the highly unstable nitroglycerin, and the nitroglycerin greatly increases the energy density of the resulting powder. Double base powders burn faster than single base powders of the same shape, and in general the higher the nitroglycerin content of a powder, the faster the burn rate.
Solid propellants--moderated explosives
A recent topic of research has been in the realm of caseless cartridges. In a caseless cartridge, the propellant is cast as a single solid grain, with the priming compound placed in a hollow at the base, and the bullet glued to the front. Since the single propellant grain is so large (most smokeless powders have grain sizes around 1 mm, a caseless grain will be perhaps 7 mm diameter and 15 mm long) the relative burn rate must be much higher. To reach this rate of burning, caseless propellants often use moderated explosives, such as RDX. While there is at least one experimental military rifle (the H&K G11) and one commercial rifle (made by Voere) that use caseless rounds, they are not having much success. The caseless ammunition is by necessity not reloadable (a major disadvantage in civilian markets where reloading is common) and the exposed propellant makes the rounds less rugged. Also, the case in a standard cartridge serves as a seal, keeping gas from escaping the breech. Caseless arms must use a more complex self sealing breech, which increases the design and manufacturing complexity. Another problem peculiar to autoloading arms firing caseless rounds is the problem of rounds "cooking off". This is caused by residual heat from the chamber heating the round in the chamber to the point where it ignites, causing an unintentional discharge. Belt fed machine guns designed for high volumes of fire are designed to fire from an open bolt, which means that the round is not chambered until the trigger is pulled, and so there is no chance for the round to cook off before the operator is ready. Open bolt designs are generally undesirable for anything but belt fed machineguns and pistol sized submachineguns. The reason is that the mass of the bolt moving forward causes the gun to lurch in reaction, which significantly reduces the accuracy of the gun. Since one of the motivating factors for the use of caseless rounds is to increase the rate of fire to the degree that several shots can be fired to the same point of aim, anything that reduces the accuracy of those first shots would be counterproductive. Cased ammunition serves as a heat sink to both carry heat away from the chamber after firing and to cool the chamber when chambered, reducing the risk of cookoff.
Load density and consistency, note low density detonation
Load density is the percentage of the space in the cartridge case that is filled with powder. In general, loads close to 100% density (or even loads where seating the bullet in the case compresses the powder) ignite and burn more consistently than lower density loads. In cartridges that survived from the black powder cartridge era (examples being .45 Colt, .45-70 Government) the case is much larger than needed to hold the maximum charge of high density smokeless powder. This allows the powder to shift in the case, piling up near the front or near the back of the case. This can cause significant variations in burning rate, as powder near the rear of the case will ignite rapidly, but powder near the front of the case will ignite slower. This change has less impact with fast powders, so high capacity, low density cartridges generally deliver best accuracy with the fastest appropriate powder, although this keeps the total energy low due to the sharp, high pressure peak. Magnum pistol cartridges reverse this power/accuracy tradeoff by using lower density powders that give high load density, and a broad pressure curve. The downside is the increased recoil and muzzle blast from the high powder mass and high muzzle pressure. The advantage is that the magnum pistol rounds generate accuracy comparable to a good hunting rifle, and energy sufficient to take medium game at ranges out to 100 yards (100 meters) and beyond.
Most rifle cartridges have a high load density with the appropriate powders. Rifle cartridges tend to be bottlenecked, with a wide base narrowing down to a smaller diameter to hold a light, high velocity bullet. These cases are designed to hold a large charge of low density powder, for an even broader pressure curve than a magnum pistol cartridge. These cases require the use of a long rifle barrel to extract their full efficiency, although they are also chambered in rifle-like pistols (single shot or bolt action) with barrels of 10 to 15 inches (25 to 38 cm).
One unusual phenomenon occurs when high density powders are used in large capacity rifle cases. Small charges of powder, unless held tightly near the rear of the case by wadding, can apparently detonate when ignited. The mechanism of this phenomenon is not well known, and generally it is not encountered except when loading very low velocity subsonic rounds for rifles. These rounds generally have velocities under 500 ft/s (195 m/s) and are used for indoor shooting or pest control where the power and muzzle blast of a full power round is not needed or desired.
Chamber
Straight vs. bottleneck
Straight walled cases were the standard from the beginnings of cartridge arms. With the low burning speed of black powder, the best efficiency was achieved with large, heavy bullets, so the bullet was the largest practical diameter. The large diameter allowed a short, stable bullet with high weight, and the maximum practical bore volume to extract the most energy possible in a given length barrel. There were a few cartridges that had long, shallow tapers, but these were generally an attempt to use an existing cartridge to fire a smaller bullet with a higher velocity and lower recoil. With the advent of smokeless powders, it was possible to generate far higher velocities by using a slow smokeless powder in a large volume case, pushing a small, light bullet. The .30-30 Winchester was one of the first rounds to be designed to use smokeless powder, and it has a distinct shoulder that closely resembles modern cartridges, even though it dates back to the 1890s. Modern cases have shorter necks, sharper shoulder angles, and a rimless design which give better efficiency and feeding, but the .30-30 cartridge and the Winchester model 1894 rifle still account for more game in North America than any other rifle and cartridge combination.
Aspect ratio and consistency, functional considerations
When selecting a rifle cartridge for maximum accuracy, a short, fat cartridge will generally yield a higher efficiency and consistency than a long, thin cartridge (part of the reason for a bottlenecked design). The ideal shape would be something near spherical, but this would be impractical to build. Target and varmint hunting rounds require the greatest accuracy, so tend to be short and fat, with sharp shoulders on the case. These cartridges do have disadvantages, however. The fat rounds take up a lot of space in a magazine, and the sharp shoulders do not feed easily out of a magazine and into the chamber. For this reason, rounds that do not require the utmost accuracy, but do need to be chambered in repeating arms (such as military rifles) tend to have longer cases with shallower shoulder angles.
Friction and inertia
Static friction and ignition
Since the burning rate of smokeless powder varies directly with the pressure, the initial pressure buildup has a significant effect on the final velocity, especially in cartridges with fast powders. The friction holding the bullet in the case determines how soon after ignition the bullet moves, and since the motion of the bullet increases the volume and drops the pressure, a difference in friction can change the slope of the pressure curve. In general, a tight fit is desired, to the extent of crimping the bullet into the case. In straight walled rimless cases such as the .45 ACP, an aggressive crimp is not possible since the case is held in the chamber by the mouth of the case, but sizing the case to allow a tight interference fit with the bullet can give the desired result.
Kinetic friction
The bullet must tightly fit the bore to seal the high pressure of the burning gunpowder. This tight fit generates a large quantity of friction. The friction of the bullet in the bore does have a slight impact on the final velocity, but that is generally not much of a concern. Of greater concern is the heat that is generated due to the friction. At velocities of about 1000 ft/s (390 m/s) lead begins to melt and deposit in the bore. This lead buildup constricts the bore, increasing the pressure and decreasing the accuracy of subsequent rounds, and is difficult to scrub out without damaging the bore. Rounds used at velocities up to 1500 ft/s (585 m/s) can use wax lubricants on the bullet to reduce lead buildup. At velocities over 1500 ft/s (585 m/s) nearly all bullets are jacketed in copper or a similar alloy that is soft enough not to wear on the barrel but melts at a high enough temperature to reduce buildup in the bore. Copper buildup does begin to occur in rounds that exceed 2500 ft/s (975 m/s), and a common solution is to impregnate the surface of the bullet with molybdenum disulfide lubricant. This reduces copper buildup in the bore and results in better long term accuracy.
The role of inertia
In the first few inches (centimeters) of travel down the bore, the bullet reaches a significant percentage of its final velocity, even for high capacity rifles with slow burning powder. The acceleration is on the order of tens of thousands of gravities, so even a projectile as light as 40 grains (2.6 g) can provide hundreds of pounds (over 1000 N) of resistance due to inertia. Changes in bullet mass therefore have a huge impact on the pressure curves of smokeless powder cartridges, unlike black powder cartridges. This makes loading or reloading smokeless cartridges require high precision equipment and carefully measured tables of load data for given cartridges, powders, and bullet weights.
Pressure curve
Missing image
Int_bal_graph.png
Image:Int_bal_graph.png
This is a graph of a simulation of the 5.56mm NATO round being fired from a 20 inch barrel. The horizontal axis represents time, the vertical axis represents pressure (green line), bullet travel (red line), and bullet velocity (light blue line). The values shown at top are peak values.
Peak vs. area
Energy is defined as a force exerted over a distance; for example, the force required to lift a one pound weight one foot against the pull of gravity defines a foot-pound of energy (lifting one newton one meter gives one newton-meter of energy). If we were to modify the graph to reflect pressure as a function of distance, the area under that curve would be the total energy imparted to the bullet. From this it can be seen that the way to increase the energy of the bullet is to increase the area under that curve, either by raising the average pressure or increasing the distance the bullet travels under pressure (in other words lengthen the barrel).
Pressure vs. distance traveled
Int_bal_burn_rate_changes.png
This graph shows different pressure curves for powders with different burn rates. The leftmost graph is the same as the large graph above. The middle graph shows a powder with a 25% faster burn rate, and the rightmost graph shows a powder with a 20% slower burn rate. Using a faster burning powder does yield a higher velocity, but at the cost of a much higher peak pressure. A slower burning powder gives a wider curve, but the lower pressure peak drops the energy. If the case could handle the increase in volume, a larger quantity of slower powder might give the same pressure peak as the medium rate powder, but with a wider curve to increase the overall energy transferred to the bullet.
Propellant burnout
Another issue to consider when choosing a powder burn rate is the time the powder takes to completely burn vs. the time the bullet spends in the barrel. Since the burn rate of nitrocellulose based powders increases with increasing pressure, this can be a very difficult interaction to guess, and requires careful testing with gradual changes. Looking carefully at the left graph, there is a change in the curve at about 0.8 ms. This is the point at which the powder is completely burned, and no new gas is created. With a faster powder, burnout occurs earlier, and with the slower powder it occurs later. Propellant that is unburned when the bullet reaches the muzzle is wasted--it adds no energy to the bullet, but it does add to the recoil and muzzle blast. For maximum power, the powder should burn until just short of the muzzle.
Since smokeless powders burn, not detonate, the reaction can only take place on the surface of the powder. Smokeless powders come in a variety of shapes, which serve to determine how fast they burn, and also how the burn rate changes as the powder burns. The simplest shape is a ball powder, which is in the form of round or slightly flattened spheres. Ball powder has a fairly small surface area to volume ratio, so it burns fairly slowly, and as it burns its surface area to volume ratio decreases. This means as the powder burns, the burn rate slows down. To some degree this can be negated by the use of retardant coatings on the surface of the powder, which slows the initial burn rate further and flattens out the rate of change. Ball powders are generally formulated as slow pistol powders or fast rifle powders. Flake powders are in the form of flat, round flakes which have a very high surface area to volume ration. Flake powders have a nearly constant rate of burn, and are usually formulated as fast pistol or shotgun powders. The last common shape is an extruded powder, which is in the form of a cylinder, sometimes hollow. Extruded powders generally have a lower ratio of nitroglycerine to nitrocellulose, and are often progressive burning--that is they burn at a faster rate as they burn. Extruded powders are generally medium to slow rifle powders.
Muzzle pressure concerns
From the pressure graphs is can be seen that the residual pressure in the barrel as the bullet exits is quite high--in this case, over 16kpsi. While lengthening the barrel or reducing the amount of propellant gas will reduce this pressure, this is often not possible due to issues of firearm size and minimum required energy. Short range target guns are generally chambered in .22 Long Rifle or .22 Short, which have very tiny powder capacities and little residual pressure. When higher energies are required for long range shooting, hunting or antipersonnel use, high muzzle pressures are a necessary evil. With this high muzzle pressures come increased flash and noise from the muzzle blast, and due to the large powder charges used, higher recoil--recoil includes the reaction for not just the bullet but also for the powder mass.
General concerns
Bore diameter and energy transfer
A firearm in many ways is like a piston engine on the power stroke. There is a certain amount of high pressure gas available, and energy is extracted from it by making the gas move a piston--in this case, the projectile is the piston. The swept volume of the piston determines how much energy can be extracted from the given gas. The more volume swept by the piston, the lower the exhaust pressure (in this case the muzzle pressure). Any remaining pressure is lost energy.
To extract the maximum amount of energy, then, the swept volume is maximized. This can be done in one of two ways--increasing the length of the barrel, or increasing the diameter of the projectile. Increasing the barrel length will linearly increase the swept volume, while increasing the diameter will increase the swept volume as the square of the diameter. Since barrel length is limited by practical concerns to about arm's length for a rifle, and much shorter for a handgun, increasing bore diameter is the normal way to increase the efficiency of a cartridge. The limit to bore diameter is generally the sectional density of the projectile (see external ballistics). Larger diameter bullets have much more drag, and so they lose energy very quickly after exiting the barrel. In general, most handguns use bullets between .357 (9mm) and .45 (11.5mm) caliber, while most rifles generally range from .223 (5.56mm) to .32 (8mm) caliber. There are many exceptions, of course, but bullets in the given ranges provide the best general purpose performance. Handguns use the larger diameter bullets for greater efficiency in short barrels, and tolerate the long range velocity loss since handguns are seldom used for long range shooting. Handguns that are used for long range shooting are generally closer to shortened rifles than other handguns.
Ratio of propellant to projectile mass
Another issue when choosing or developing a cartridge is the issue of recoil. The recoil is not just the reaction from the projectile being launched, but also from the powder gas, which will exit the barrel with a velocity even higher than that of the bullet. For handgun cartridges, with large bullets and small powder charges (a 9mm, for example, might use 5 grains of powder and a 115 grain bullet) this is not a significant force; for a rifle cartridge (a .22-250 Remington using 40 grains of powder and a 40 grain bullet) the powder charge can make for the majority of the recoil force. There is a solution to the recoil issue, though it is not without cost. A muzzle brake or recoil compensator is a device which redirects the powder gas at the muzzle, usually up and back. This acts like a rocket, pushing the muzzle down and forward. The forward push helps negate the feel of the projectile recoil by pulling the firearm forwards, and the downward push helps counteract the rotation imparted by the fact that most firearms have the barrel mounted above the center of gravity. Overt combat guns, large bore, high powered rifles, long range, handguns chambered for rifle ammunition, and action shooting handguns designed for accurate, rapid fire all benefit from muzzle brakes. The high powered firearms use the muzzle brake mainly for recoil reduction, which reduces the battering of the shooter by the severe recoil. The action shooting handguns redirect all the energy up, to counteract the rotation of the recoil, and make following shots faster by leaving the gun on target. The disadvantage of the muzzle brake is a longer, heavier barrel, and a large increase in sound levels behind the muzzle of the rifle. Shooting firearms without hearing protection can eventually damage the operator's hearing. Shooting firearms with muzzle brakes with no hearing protection can damage it far faster. Even with adequate hearing protection, the increased pounding of the redirected muzzle blast can quickly cause headaches.
Powder to projectile weight ratio also touches on the subject of efficiency. In the case of the .22-250 Remington, more energy goes into propelling the powder gas than goes into propelling the bullet. The .22-250 pays for this by requiring a large case, with lots of powder, all for a fairly small gain in velocity and energy over other .22 caliber cartridges.
Accuracy and bore characteristics
Nearly all small bore firearms, with the exception of shotguns, have rifled barrels. The rifling imparts a spin on the bullet, which keeps it from tumbling in flight. The rifling is usually in the form of sharp edged grooves cut as helixes along the axis of the bore, anywhere from 2 to 16 in number. The remaining, tighter, or "higher" [serious grammatical breakdown] parts are called "lands". Another system, polygonal rifling, gives the bore a polygonal cross section. Polygonal rifling is not very common, used by only a few European manufacturers. The companies that use polygonal rifling claim greater accuracy, lower friction, and less lead and/or copper buildup in the barrel. Traditional land and groove rifling is used in most competition firearms, however, so the advantages of polygonal rifling are unproven. There are three common ways of rifling a barrel. The most basic is to use a single point cutter, drawn down the bore by a machine that carefully controls the rotation of the cutting head relative to the barrel. This is the slowest process, but as it requires the simplest equipment, it is often used by custom gunsmiths and can result in superbly accurate barrels. The next method is button rifling. This method uses a die with a negative image of the rifling cut on it. This die is drawn down the barrel while carefully rotated, and it swages the inside of the barrel. This "cuts" all the grooves at once (it doesn't really cut metal) and so is faster than cut rifling. Detractors claim that the process leaves considerable residual stress in the barrel, but world records have been set with button rifled barrels, so again there is no clear advantage. The last method used is hammer forging. In this process, a slightly undersized bored barrel is placed around a mandrel that contains a negative image of the entire length of the rifled barrel. The barrel and mandrel are rotated and hammered by power hammers, which forms the inside of the barrel all at once. This is the fastest (and in the long run, cheapest) method of making a barrel, but the equipment is prohibitively expensive for all but the largest gunmakers. Hammer forged barrels are strictly mass produced, so they are generally not capable of top accuracy as produced, but with some careful hand work, they can be made to shoot far better than most shooters are capable of.
The purpose of the barrel is to provide a consistent seal, allowing the bullet to accelerate to a consistent velocity. It must also impart the right spin, and release the bullet consistently, perfectly concentric to the bore. The residual pressure in the bore must be released symmetrically, so that no side of the bullet receives any more or less push than the rest. The muzzle of the barrel is the most critical part, since that is part that controls the release of the bullet. Some rimfires and airguns actually have a slight constriction, called a choke, in the barrel at the muzzle. This guarantees that the bullet is held securely just before release.
To keep a good seal, the bore must be a very precise, constant diameter, or have a slight decrease in diameter from breach to muzzle. Any increase in bore diameter will allow the bullet to shift. This can cause gas to leak past the bullet, affecting the velocity, or cause the bullet to tip so that it is no longer perfectly coaxial with the bore. High quality barrels are lapped to remove any constrictions in the bore which will cause a change in diameter. The lapping process uses a lead "slug" that is slightly larger than the bore and covered in fine abrasive compound to cut out the constrictions. The slug is passed from breach to muzzle, so that as it encounters constrictions, it cuts them away, and does no cutting on areas that are larger than the constriction. Many passes are made, and as the bore becomes more uniform, finer grades of abrasive compound are used. The final result is a barrel that is mirror smooth and with a consistent or slightly tapering bore. The hand lapping technique uses a wooden or soft metal rod to pull or push the slug through the bore, while the newer firelapping technique uses specially loaded low power cartridges to push abrasive covered soft lead bullets down the barrel.
Another issue that has an effect on the barrel's hold on the bullet is the rifling. When the bullet is fired, it is forced into the rifling, which cuts or "engraves" the surface of the bullet. If the rifling is a constant twist, then the rifling rides in the grooves engraved in the bullet, and everything is secure and sealed. If the rifling has a decreasing twist, then the changing angle of the rifling in the engraved grooves of the bullet causes the rifling to become narrower than the grooves. This allows gas to blow by, and loosens the hold of the bullet on the barrel. An increasing twist, however, will make the rifling become wider than the grooves in the bullet, maintaining the seal. When a rifled barrel blank is selected for a gun, careful measurement of the inevitable variations in manufacture can determine if the rifling twist varies, and put the higher twist end at the muzzle.
The muzzle of the barrel is the last thing to touch the bullet before it goes into ballistic flight, and as such has the greatest potential to disrupt the bullet's flight. The muzzle must allow the gas to escape the barrel symmetrically; any asymmetry will cause an uneven pressure on the base of the bullet which will disrupt its flight. The muzzle end of the barrel is called the crown, and it is usually either beveled or recessed to protect it from bumps or scratches that might affect accuracy. A sign of a good crown will be a symmetric, star shaped pattern on the muzzle end of the barrel, formed by soot deposited as the powder gasses escape the barrel. If the star is uneven, then it is a sign of an uneven crown, and an inaccurate barrel.
Before the barrel can release the bullet in a consistent manner, it must grip the bullet in a consistent manner. The part of the barrel between where the bullet exits the cartridge and engages the rifling is called the throat, and the length of the throat is the freebore. In some firearms, the freebore is all but nonexistent--the act of chambering the cartridge forces the bullet into the rifling. This is common in low powered rimfire target rifles. The placement of the bullet in the rifling ensures that the transition between cartridge and rifling is quick and stable. The downside is that the cartridge is firmly held in place, and attempting to extract the unfired round can be difficult, to the point of even pulling the bullet from the cartridge in extreme cases. With high powered cartridges, there is an additional disadvantage to a short freebore. A significant amount of force is required to engrave the bullet, and this additional resistance can raise the pressure in the chamber by quite a bit. To mitigate this effect, higher powered rifles tend to have more freebore, so that the bullet is allowed to gain some momentum and the chamber pressure is allowed to drop slightly before the bullet engages the rifling. The downside is that the bullet hits the rifling when already moving, and any slight misalignment can cause the bullet to tip as it engages the rifling, which will in turn mean that the bullet does not exit the barrel coaxially. The amount of freebore is a function of both the barrel and the cartridge. The manufacturer or gunsmith who cuts the chamber will determine the amount of space between the cartridge case mouth and the rifling. Setting the bullet further forward or back in the cartridge can decrease or increase the amount of freebore, but only within a small range. Careful testing by the ammunition loader can optimize the amount of freebore to maximize accuracy while keeping the peak pressure within limits.
Autoloading firearms
Autoloading firearms (this term covers automatic and semiautomatic firearms) harness some of the energy of firing to load a new round from an ammunition feeding device. This loading generally involves extracting and ejecting the fired cartridge case, obtaining a new cartridge from a clip, magazine or belt, cocking the hammer or striker, and securing the new cartridge in the chamber, ready to fire. Automatic firearms will, if the trigger remains depressed, also fire the new round upon finishing the cycle. There are a number of different mechanisms to harness the energy required, and they vary in complexity and suitability for different purposes.
One issue that applies to all autoloading mechanisms is the mass of the reciprocating components. In all cases the firing energy is harnessed to open the mechanism, and a spring or springs are used to close the mechanism. The spring must have sufficient energy to close the action and perform all tasks that are involved in that, but the spring itself does very little to keep the action closed. The energy that powers the action is only available for the time it takes for the projectile to leave the barrel, or less. This energy is converted to kinetic energy in the reciprocating parts, and the amount of energy required to propel those parts to the required velocity is what is needed for reliable operation with a given powder, bullet, and cartridge combination. Since changing the mass of the firearm parts is out of the scope of the average user (this amounts to a redesign of the firearm in most cases) it is up to the user to select ammunition that will allow the firearm to function.
Blowback firearms
The simplest autoloading firearms are known as blowback actions. In a blowback gun, the bolt that holds the cartridge in the chamber is held in place by a spring called the recoil spring. Upon firing, the same pressure that pushes the bolt forward pushes the case backwards. The same effect that operates the Blish lock operates between the cartridge and the chamber walls, which serves to hold the case in the breach for a small amount of time. This effect is not enough to keep it there, however, and the case pushes out of the chamber, moving the bolt with it. The bolt is far more massive than the bullet, so the bolt moves only a very short distance before the bullet exits the barrel and the pressure drops to ambient. In this short distance, however, the bolt has gained sufficient momentum to eject the cartridge, cock the hammer, and compress the recoil spring. The compressed recoil spring then forces the bolt back forward, chambering a new round. One interesting advantage of the blowback action is the fact that an extractor to pull the cartridge out of the chamber is not required during firing, since the cartridge is pushing the bolt out. Since extractors tend to be one of the more delicate parts of most firearms designs, this helps increase reliability under extreme conditions.
Blowback actions are very simple and inexpensive to build. They are also generally very reliable. Most semiautomatic rimfires and many submachine guns are blowback actions. The disadvantage of the blowback action is the requirement that the bolt be so much heavier than the bullet. This is why its use is generally restricted to handguns in calibers of 9mm Parabellum or smaller, and in carbines and submachine guns to calibers of .45 ACP and smaller.
There are a number of actions that are in essence blowback designs, but use a variety of approaches to reduce the velocity of the bolt so that a combination of heavier bullets and lighter bolts may be used. These are called delayed blowback actions. Techniques involve using a separate Blish lock mechanism, a piston that fills with powder gas to push the bolt closed, and a two part, cammed bolt that uses the first bit of the cartridges movement to move the main body of the bolt back a large distance. These designs allow slightly higher powered cartridges to be used with no additional modifications. The cammed bolt design has been used with high powered rifle cartridges with some success, but also requires the addition of a fluted chamber to reduce the Blish lock effect, which would otherwise blow the back out of the cartridge.
Gas operated firearms
Gas operated firearms differ from blowback actions in that the bolt is solidly locked into the breach of the barrel at the time of firing. This locking can be done with a rotating cam, or a lateral bolt, or any number of other ways. In this, gas operated firearms are not significantly different than manually operated bolt, pump, or lever action designs. What makes a difference is how energy is provided to unlock and open the bolt. At some point in the barrel, there is a small hole called a gas tap, which allows the high pressure gas to escape. This hole can be anywhere from just in front of the chamber to just behind the muzzle. This gas is directed into a cylinder and piston arrangement (fixed cylinder and moving piston, or fixed piston and moving cylinder, both have been used). The movement of the piston or cylinder is captured to provide the energy to operate the action.
The concerns when making ammunition for gas operated firearms center around the gas tap location, and the operating parameters of the cylinder and piston. The pressure in the barrel as the bullet passes the gas tap must be within a certain range of pressure, and the pressure must persist for a certain amount of time. Too little, and there will not be enough inertia gathered to cycle the action; too much, and parts will move to fast to operate reliably and the extra inertia will greatly increase wear and tear. The formulation of the powder must also be considered. Some powders produce more soot than others, and this soot can build up in the gas tap or piston and cylinder area and cause malfunctions. The gas tap is also subject to fouling from lead buildup in guns that can fire non-jacketed bullets.
A case in point is the original M-16 rifle, which is a gas operated firearm. It was designed to use an IMR type powder, which had certain burn characteristics. The U.S. military, against the advice of the designer, switched to a ball type powder, with a different burn rate which yielded higher velocities. This also altered the pressure at the gas tap enough to increase the bolt velocity enough to tear apart cartridges, cause misfeeds, and generally turn what was a reliable rifle into a nightmare situation.
Gas action firearms require fairly high pressures to operate, and so are generally found in high powered hunting or military rifles, and magnum pistols.
Recoil operated firearms
The last common type of autoloading firearm is the recoil operated type. Recoil operated firearms, as the name implies, rely on the recoil generated by firing, rather than the gas pressure used by blowbacks and gas operated firearms. Recoil operated firearms are locked breech firearms, and come in two varieties, short recoil and long recoil operation.
Long recoil operation is fairly rare, and used in heavily recoiling firearms like shotguns, automatic grenade launchers, and heavy machine guns. In a long recoil action, the barrel and bolt recoil as a unit, separate from the rest of the gun, compressing two recoil springs on the way. When they bolt and barrel reach the full rearward extent, the bolt locks back, and the barrel springs forward, propelled by one of the recoil springs. When the barrel reaches the full forward position, then the bolt is released, and moves forward under the power of the other recoil spring.
Short recoil operation is almost exclusively found in pistols, and it is the most common action type in pistols of 9mm Parabellum or greater. In a short recoil action, the barrel and bolt travel backwards for only a short distance before the barrel is unlocked (usually by a camming action) and the bolt continues on its own. When the bolt returns to the barrel, still locked in the rearward position, the barrel is cammed back into the locked position, and the gun is again ready to fire.
Ammunition intended for recoil operated guns needs to generate a sufficient amount of recoil to move the bolt and barrel back at the required velocity. This means that bullet weights are the primary concern, since the required energy generally needs a moderately heavy bullet for the caliber propelled close to the maximum kinetic energy. To cut the bullet mass in half would require doubling the velocity, which would be doubling the kinetic energy as well. It is rare to find a short recoil gun that can handle a minimum bullet mass that is less than half the maximum mass, and most have a much smaller range of weights.nl:Inwendige ballistiek