M79 Grenade Launcher
This was to be totally replaced by the underbarrel M203 but is still used today in some purposes.
If the ammunition had used a simple cartridge case, it would not be possible to fire the projectile from such a light weapon, since the breech pressure and muzzle velocity would be insane with normal powders. An extremely slow powder designed to keep the breech pressure down would neither ignite properly nor burn properly, so this was not an option.
The solution was extremely clever: a small charge of conventional powder is held in a small inner compartment of closed cylindrical form adjacent to the primer. This connects to the main body of the cartridge case with a series of bleed holes. The peak pressure is retained by the inner compartment, and the gas that enters the main body of the cartridge case does so at a much lower, more manageable pressure. This enables a lightweight shoulder fired weapon to fire 40 mm ammunition at a leisurely velocity.
Not to forget the China Lake NATIC, A pump-action variant based on the M79 designed to be used by the US Navy SEALs but got turned down?.