What is to become reduced in diameter, or changed in shape, by passing from a larger to a smaller part of the bore of the barrel; -- said of a bullet when fired from a gun, pistol, or other firearm called?

slug — To become reduced in diameter, or changed in shape, by passing from a larger to a smaller part of the bore of the barrel; -- said of a bullet when fired from a gun, pistol, or other firearm. (Webster's 1913.)