Smoothbore
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Smoothbore refers to a firearm which does not have a rifled barrel. Rifling imparts a spin to the projectile, which stabilizes it and prevents it from tumbling. Early firearms did not have rifling, and had to fire spherical projectiles, so the random tumbling impacted the accuracy as little as possible. But like a curveball, even a relatively smooth sphere will curve when rotating on any axis not parallel to the direction of travel. In the 18th century, the standard infantry arm was the smoothbore musket; by the 19th century, rifled barrels became the norm, because they could fire elongated projectiles, which provided better external ballistics.
Some firearms are still made smoothbore. The most common of these is the shotgun. Firing mutiple, non coaxial projectiles out of a rifled barrel would result in an "O" shaped pattern, with a very rapid spread, a high projectile density on the periphery, and a low projectile density in the interior. This is the exact opposite of the desired even distribution, with a fairly even number of pellets across a carefully expanding region.
The other type of smoothbore that has come into use over the last part of the 20th century is the tank gun. Practical rifling can only stabilize projectiles of a certain length to diameter ratio. To reliably penetrate the thick armor of modern armored vehicles, a very long, thin kinetic energy projectile is required—too long to stabilize with rifling. These round are instead formed into a dart shape, using fins for stabilization (see kinetic energy penetrator for information on how this works). With the fins for stability, rifling is no longer needed and in fact the spin imparted by rifling will degrade the accuracy of a finned projectile. Because of the increasing use of the highly effective kinetic energy penetrator projectiles by tank guns, most modern tanks have smoothbore barrels.