1431. Mouth plugs. With separate-case ammunition, it was formerly the practice to seal the case with a brass mouth cup. This was in the form of a brass bowl, the sides of which fitted inside of the case. The object in its use was twofold: first, to form a gas seal in the case to prevent escape of the gases past the rotating band — also to keep gases from breaking cases by outside pressure on the case, before the projectile started to engrave the band in the grooves — and, second, to make the case an air-tight container for the powder charge.
It has been shown that the gas escape is insufficient to require this form of seal, and as the brass mouth cup had a tendency to break up and boomerang back, endangering personnel, cork plugs have been substituted.