Terminal ballistics
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Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a kinetic energy projectile when it hits its target. It is often referred to as stopping power when dealing with human or other living targets. Terminal ballistics is relevant for both small caliber projectiles as for large caliber projectiles (fired from artillery). The study of extremely high velocity impacts is still very new and is as yet mostly applied to spacecraft design.
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Small caliber terminal ballistics
Classes of bullet
There are three basic classes of bullet: ones that are designed for maximum accuracy at varying ranges, ones that are designed to inflict maximal damage to a target by optimizing the depth to which the bullet penetrates, and ones that are designed to maximize damage to a target by penetrating as deeply as possible.
Bullets for target shooting
For short range target shooting on ranges up to 50 meters (55 yd) aerodynamics is relatively unimportant and velocities are low. As long as the bullet is balanced so it doesn't tumble, the aerodynamics are unimportant. For shooting at paper targets, the best bullet is one that will punch a perfect hole through the target. These bullets are called wadcutters, and they have a very flat front, often with a relatively sharp edge along the perimeter. The flat front punches out a large hole in the paper, close to if not equal to the full diameter of the bullet. This allows for easy, unambiguous scoring of the target. Since cutting the edge of a target ring will result in scoring the higher score, fractions of an inch are important. In magazine fed pistols, the square shape of a wadcutter will often not feed reliably. To address this, the semiwadcutter was developed. The semiwadcutter consists of a conical section that comes to a smaller flat, and a thin sharp shoulder at the base of the cone. The flat point punches a clean hole, and the shoulder opens the hole up cleanly. For steel targets, the concern is to provide enough force to knock over the target, but to minimize the damage to the target. A soft lead bullet, or a jacketed hollow point bullet or soft point bullet will flatten out on impact (if the velocity at impact is sufficient to make it deform), spreading the force over a larger area of the target, allowing more total force to be applied without damaging the steel target.
There are also specialized bullets designed specifically for use in long-range precision target shooting with high-powered rifles; the designs vary somewhat from manufacturer to manufacturer, but all are based on the MatchKing bullets introduced by the Sierra Bullet Company around 1963. Based on research done in the 1950s by the US Air Force, in which it was discovered that bullets are more stable in flight for longer distances and more resistant to crosswinds if the center of gravity is somewhat to the rear of the center of pressure, the MatchKing bullet (which is still in wide use and holds many records) is a hollowpoint design with a tiny aperture in the jacket at the point of the bullet and a hollow air space under the point of the bullet, where previous conventional bullets had had a lead core that went all the way up to the point. Other designs from other manufacturers may be anything from close copies of the MatchKing design to hollowpoint bullets with a deep, wide cavity containing a long, slender, pointed plastic or aluminum plug. In all these cases, the bullet is designed to have its center of gravity to the rear of its center of pressure. MatchKing type hollowpoint bullets, as contrasted with hollowpoint bullets intended for hunting or police use, are not designed to flatten out on impact; this makes them a relatively poor choice for hunting, as they tend to perform erratically and unpredictably upon entering an animal's body--they may tumble, or break apart, thought most often they punch straight through making a narrow wound that usually does not cause death quickly (as full metal jacket ammunition normally does). The US military now issues ammunition to snipers that use bullets of this type. In 7.62 x 51 mm NATO, M852 Match and M118LR ammunition are issued, both of which use Sierra MatchKing bullets; in 5.56 x 45 mm NATO, those US Navy and US Marine snipers who use accurized M16 type rifles are issued the Mk 262 Mod 0 cartridge developed jointly by Black Hills Ammunition and Crane Naval Special Warfare Center, using a bullet manufactured by the Nosler company that is very similar to a Sierra MatchKing bullet.
In the mid 1990s, the US military Adjutant General's Office issued a legal opinion holding that the Sierra MatchKing bullet, despite being a hollowpoint design, is not designed specifically to cause greater damage or suffering in a human target, and in fact normally does not create a wound readily distinguishable from wounds caused by conventional full metal jacket bullets, and is therefore in their opinion legal under the Hague Convention for use in war.
Bullets for maximum penetration
For use against armored targets, or large, tough game animals, penetration is the most important consideration. Focusing the largest amount of momentum on the smallest possible area of the target provides the greatest penetration. Bullets for maximum penetration are designed to resist deformation upon impact, and usually are made of lead that is covered in a copper, brass, or mild steel jacket (some are even solid copper or bronze alloy). The jacket completely covers the front of the bullet, although often the rear is left with exposed lead (this is manufacturing consideration, the jacket is formed first, and the lead is swaged in from the rear). For penetrating substances significantly harder than jacketed lead, the lead core is supplemented with or replaced with a harder material, such as hardened steel. Military armor piercing small arms ammunition is made with a steel core, and the current NATO 5.56 mm SS109 bullet contains a steel tip in front of a lead core, for enhanced armor penetration. For larger, higher velocity calibers, such as tank guns, hardness is of secondary importance to density, and are normally sub-caliber projectiles made from depleted uranium fired in a light aluminum or magnesium alloy sabot. Oddly, many modern tank guns are smoothbore, not rifled. This is because practical rifling twists can only stabilize projectiles with a length to diameter ratio of up to about 5:1, and also because the rifling adds friction and reduces the velocity it is possible to achieve. To get the maximum force on the smallest area, anti-tank rounds have aspect ratios of 10:1 or more. Since these cannot be stabilized by rifling, they are built instead like large darts, with fins providing the stabilizing force, negating the need for rifling. These subcaliber rounds are held in place in the bore by sabots. The sabot is a light material that transfers the pressure of the charge to the penetrator, then is discarded when the round leaves the barrel.
Bullets for controlled penetration
The final category of bullets are those intended to maximize damage to living targets. These are used primarily for hunting and civilian antipersonnel use; they are not generally used by the military, since the use of expanding bullets in international conflicts is prohibited by the Hague Convention. These bullets are designed to increase their surface area upon impact, thus avoiding greater drag and limiting the travel through the target. A desirable side effect is that the expanded bullet makes a larger hole, increasing tissue disruption and speeding incapacitation. In some applications, preventing exit from the rear of the target is also desirable. A bullet which penetrates through-and-through tends to cause more profuse bleeding, allowing a game animal to be bloodtrailed more easily. On the other hand, a perforating bullet can then continue on (likely not coaxial to the original trajectory due to target deflection) and might cause unintended damage or injury. Frangible bullets, made of tiny fragments held together by a weak binding, are often sold as an "ultimate" expanding bullet, as they will increase their effective diameter by an order of magnitude. When they work, they work extremely well, causing massive trauma to the target. On the other hand, when they fail, it is due to underpenetration, and the damage to the target is superficial and leads to very slow incapacitation.
Flat point bullets
The simplest maximum disruption bullet is one with a wide, flat tip. This increases the effective surface area, as rounded bullets can allow tissues to "flow" around the edges. It also increases drag, which decreases the depth to which the bullet penetrates. Flat point bullets, with fronts of up to 90% of the overall bullet diameter, are usually designed for use against large or dangerous game. They are often made of unusually hard alloys, are longer and heavier than normal for their caliber, and even include exotic materials such as tungsten to increase their sectional density. These bullets are designed to penetrate deeply through muscle and bone, while causing a wound channel of nearly the full diameter of the bullet. These bullets are designed to penetrate deeply enough to reach vital organs from any shooting angle. One of the common hunting applications of the flat point bullet is large game such as bear hunted with a handgun in a .44 Magnum or larger caliber. The disadvantage of flat point bullets is the reduction in aerodynamic performance; the flat point induces much drag, leading to significantly reduced velocities at long range.
Expanding bullets
More effective on lighter targets are the expanding bullets, the hollow point bullet and the soft point bullet. These are designed to use the hydraulic pressure of muscle tissue to expand the bullet. This process is called mushrooming, as the ideal result is a shape that resembles a mushroom—a cylindrical base, topped with a wide surface where the tip of the bullet has peeled back to expose more area. A copper-plated hollowpoint loaded in a .44 Magnum, for example, with an original weight of 240 grains (16 g) and a diameter of 0.43 inch (11 mm) might mushroom on impact to form a rough circle with a diameter of 0.70 inch (18 mm) and a final weight of 239 grains (15 g). This is excellent performance; almost the entire weight is retained, and the frontal surface area increased by 165%. Penetration of the hollowpoint would be less than half that of a similar nonexpanding bullet, and the resulting wound cavity would be much wider.
Frangible bullets
The last category of expanding bullets are the frangible bullets. These bullets are designed to break up on impact, which results in a huge increase in surface area. The most common of these bullets are made of small diameter lead pellets, placed in a thin copper shell and held in place by an epoxy or similar binding agent. Upon impact, the epoxy shatters and the copper shell opens up, much like a hollowpoint. The individual lead balls then spread out in a wide pattern, and due to their low mass to surface area ratio, stop very quickly. Similar bullets are made out of sintered metals, which turn to powder upon impact. These bullets are usually restricted to pistol cartridges, as the nonhomogenous cores tend to cause inaccuracies that, while acceptable at short pistol ranges, are not acceptable for the typical range at which rifles are used. One interesting use of the sintered metal rounds is in shotguns in hostage rescue situations; the sintered metal round is used at near-contact range to shoot the lock mechanism out of doors. The resulting metal powder will immediately disperse after knocking out the door lock, and cause little or no damage to occupants of the room. Frangible rounds are also rumored to be used by armed security agents on aircraft. The concern is not depressurization (a bullet hole will not depressurise an airliner) but over penetration and damage to vital electrical or hydraulic lines, or injury to an innocent bystander by a bullet that travels through a target's body completely instead of stopping in the body.
Also used are bullets similar to hollowpoint bullets or soft point bullets whose cores and/or jackets are deliberately weakened to cause deformation or fragmentation upon impact. The Warsaw Pact 5.45 x 39 mm M74 assault rifle round exemplifies a trend that is becoming common in the era of high velocity, small caliber military rounds. The 5.45 x 39 mm uses a steel jacketed bullet with a 2 part core, the rear being steel and the front being lead. Upon impact, the lead deforms, bending the bullet into a slight "L" shape. This causes the bullet to tumble in the tissue, thus increasing its effective frontal surface area by traveling sideways more often than not. This does not violate the Hague Convention, as it specifically mentions bullets that expand or flatten in the body. The NATO SS109 also tends to bend at the steel/lead junction, but with its weaker jacket, it fragments into many dozens of pieces. NATO 7.62 mm ball manufactured by some countries, such as Germany and Sweden, are also known to fragment due to jacket construction.
Other bullets in use by militaries are quite back heavy, due to a long, sharp point created in an attempt to get the maximum ballistic coefficient (see external ballistics). These bullets will flip over after impact, then settle into a stable, back first orientation before stopping. The Swiss military actually redesigned their 5.56 mm assault rifle bullet to prevent this, to more fully comply with the spirit of the Hague Convention, though according to some sources the present GP90 5.56x45mm Swiss assault rifle ammunition was actually designed as an armor-piercing bullet, because in the 1980s it was perceived that the Soviets and their Warsaw Pact allies were going to issue soft body armor to infantry units on a wide basis, but after the end of the Cold War, the Bofors corporation, having spent a great deal of money on developing the new bullet, changed the sales pitch in order to sell it to the Swiss government.
It might seem that if the whole purpose of a maximum disruption round is to expand to a larger diameter, it would make more sense to start out with the desired diameter rather than relying on the somewhat inconsistent results of expansion upon impact. While there is merit to this (there is a strong following of the .45 ACP, as compared to the 0.355 in diameter 9 x 19 mm, for just this reason) there are also significant downsides. A larger diameter bullet is going to have significantly more drag than a smaller diameter bullet of the same mass, which means long range performance will be significantly degraded. A larger diameter bullet also means more space is required to store the ammunition, which means either bulkier guns or smaller magazine capacities. The common trade-off when comparing .45 ACP and 9 x 19 mm pistols is a 7 or 8 round capacity in the .45 ACP vs. a 13 to 15 round capacity in the 9x19 mm. Although several .45-caliber pistols are available with high-capacity magazines (Para Ordnance being one of the first in the late 1980s) many people find the wide grip required uncomfortable and difficult to use. Especially where the military requirement of a nonexpanding round is concerned, there is fierce debate over whether it is better to have fewer, larger bullets for enhanced terminal effects, or more, smaller bullets for increased number of potential target hits.
Selecting bullets for terminal performance
The standard medium for testing bullets for performance on tissue is ballistic gelatin. Tests have shown that properly prepared and calibrated 10% (by mass) gelatin at 4 degrees Celsius correlates very closely to observed performance in the muscle tissue of living, anesthetised swine. Performance is generally graded with two factors, the maximum depth of penetration and the size of the cavity formed in the gelatin by the bullet impact. The size of the cavity represents the distance which tissue is thrown radially outward due to "splash." The penetration represents how far into the tissue the bullet will ultimately penetrate.
Unfortunately, gelatin is a poor medium for evaluating actual effectiveness. The observed "tissue splash," usually referred to as "temporary cavitation," is not an indication of terminal performance in an animal, as gelatin has a much, much lower elastic limit than most living tissues; a force which tears a gelatin block in half may result in nothing more than slight bruising if applied to living flesh.
Penetration figures may not be accurate. Many testers do not calibrate their gelatin. The standard calibration is 85 mm of penetration when shot by a standard .177 caliber steel bb traveling at 180 m/s (590 ft/s). Uncalibrated gelatin may show a variance of up to + or - 50% from calibrated gelatin. Further, animals' skin resists penetration much more than the muscle tissue which gelatin simulates. Human skin tissue on the torso resists penetration as much as 50 mm (2 in) of muscle, and horses' skin is the equivalent of approximately 200 mm (7.9 in).
For a quick incapacitation, a hit to a vital, blood-bearing organ or the central nervous system is needed, so a bullet that will penetrate to the depth required for such a hit should be chosen. When hunting groundhogs, for example, a bullet that expands quickly to form a large cavity with minimum penetration would be the best choice. When hunting deer, a bullet which penetrates deeper is required; this can be accomplished by either limiting expansion (2 times the original width is often regarded as ideal), or by using a more powerful cartridge. For hunting bear, yet more penetration is required. The pattern is, of course, that the larger the animal, the deeper its vital organs will be located, and therefore a firearm, cartridge, and bullet type should be chosen that will be able to reach the vital organs and kill humanely.
For dangerous game especially, deep penetration depth is critical; the reason for this is that the shooter cannot always choose their shots. If a hunter finds himself staring at a deer's hindquarters, it is very unlikely that he or she will choose to fire at that deer anyway, in the hopes that their bullet will be able to reach a vital organ through several layers of muscle and gut. The better choice in that scenario would be to wait until the deer decides to turn around. A lion, however, may decide to charge at a person other than the shooter, presenting a much less than optimal shooting angle.
To hit the vital organs on a large game animal requires penetrating the thick fat and muscle tissue surrounding the chest cavity, and quite often bone as well. A hard, nondeforming bullet is often chosen, though many modern rifle calibers are quite capable of killing 1,000 lb (450 kg) elk and similar-sized animals with a deforming bullet; even the venerable .30-06 is up to the task, with a powerful enough load. Elephant hunters normally attempt to shoot for the brain, which is much smaller than the size of the elephant's head, and so must be targeted quite precisely, and require a firearm and bullet capable of punching through a foot (300 mm) or more of tough, albeit hollow, bone and reaching the brain.
Terminal ballistics for non-military defensive purposes
The rules of engagement for non-military use of firearms usually require that a life be in immediate danger for shots to be fired. Under such circumstances, the goal is to incapacitate the target as quickly as possible, to prevent the harm from being done. In most cases, the shots are fired from a handgun, which is, compared to a rifle, very much underpowered. Humans are in roughly the same class as deer sized game, and in most places, the minimum cartridge power required to hunt deer is more than twice that of the average police sidearm. Handguns are also very inaccurate in the hands of all but the best shooters, and the average defensive shooter is not an expert, and is under a great deal of stress, which further degrades accuracy. These factors combine to require extremely effective terminal ballistics to provide swift incapacitation of the target under far less than ideal circumstances.
Humans walk upright and present relatively unprotected vital organ targets from some angles, and have substantially thinner skin, so the bare minimum penetration is lower than for deer. Cross-torso shots and shots which must first penetrate an arm are relatively common in defensive shooting scenarios, however.
Bullets for use on humans are usually designed to comply with the FBI's penetration requirement of 12 to 18 inches (305 to 457 mm), which is based on the IWBA's requirement of 12.5 to 14 inches (318 to 356 mm). This is to ensure that the bullet can reach a vital blood-bearing organ or central nervous system structure from most angles. Frangible rounds, while they are sold for defensive purposes, are not well suited for the role, as they generally penetrate less than 10 inches (254 mm), and are therefore prone to failure when they must pass through nonvital tissues, such as a hand or arm. When they work, they work very well, but when they fail, they tend to fail badly.
Hollowpoint bullets normally expand most when at their highest velocity; that is, when entering the target. As they expand, they slow. Hollowpoint bullets may not expand when they strike sheet metal, glass, or bulky clothing before the target. These preliminary obstacles can either fill the hollowpoint cavity or deform the lips of the cavity. Either of these effects can prevent the high internal hydrostatic pressure necessary to make the hollowpoint round expand.
For in-depth information on the mechanisms (and misconceptions) by which bullets incapacitate living targets, see the article on stopping power.
Large caliber terminal ballistics
The purpose of firing a large caliber projectile is not always the same. For example, one might need to create disorganisation within enemy troops, create casualties within enemy troops, eliminate the functioning of an enemy tank, or destroy an enemy bunker. Different purposes of course require different projectile designs.
Many large caliber projectiles are designed to explode, releasing splinters of the casing, or subcaliber balls (this is the correct meaning of "shrapnel") or more rarely to release chemical weapons or biological weapons either on impact or when close to a target; designing an appropriate fuse is a difficult task which lies outside the realm of terminal ballistics.
Other large caliber projectiles use subprojectiles, which are released when the carrying projectile explodes close to its target. This kind of projectile also aims to create maximum casualties amongst enemy troops. A side-effect is that unexploded subprojectiles can later be found and create casualties at that time. International conventions tend to forbid or restrict the use of this type of projectile.
Standard armor-piercing projectiles use what is known as a shaped charge, where the face of the explosive which strikes the target has a deep, cone-shaped indentation in it. Shaped charges have been used ever since it was discovered that a block of high explosives with letters engraved in it created perfect impressions of those letters when detonated against a piece of metal. They operate by disintegrating a thin metal (often copper) cone, placed right against the explosive, and violently forcing the metal jet through the target's armor. Contrary to popular belief, the copper cone is not substantially heated. Rather, the massive force of the explosion is sufficient that the metal behaves like a fluid.
Shaped charges can be defended against by reactive armor which uses relatively thin plates with explosive underneath them. The explosive is detonated when the plate is struck by a warhead, and the blast is sufficient to prevent the copper jet from penetrating. The problems with reactive armor is that each plate can protect from only a single strike, and the explosion can be extremely dangerous to nearby personnel and lightly armored structures.
Shaped charge warheads are slowly being replaced by so-called "kinetic energy" projectiles, as armors become more and more advanced. Ironically, it is the most primitive projectiles that are hardest to defend against, since only equally primitive armor is effective against it. Kinetic energy missiles are usually long and narrow, like an arrow or dart, and constructed of very hard, dense material. Tungsten carbide and depleted uranium are often used. The length of the penetrator is limited by the ability of the penetrator to withstand shear forces along its length. The round typically penetrates the frontal armor belt, but turns slightly in the process, and so strikes the rear armor at a more oblique angle. This often leads to fragmentation and splashing of the penetrator (along with portions of the armor) throughout the interior of the armored volume.
Hypervelocity terminal ballistics
The study of projectile impacts with velocities greater than several kilometres per second is an area of active research. Such impacts are not yet used in military situations, but can arise from meteoric impact. The impact of extremely small, extremely fast particles is of interest in designing spacecraft to withstand erosion due to micrometeoroids.
Accelerating projectiles up to such speeds is currently difficult; light gas guns are currently the most common techniques for producing such speeds, although linear motors are also a possibility undergoing active research.
See also kinetic energy projectile.