 In fact, by trapping and slowing, and then redirecting the gases sideways, the device called a "recoil booster" actually somewhat reduces the felt recoil of the weapon, by eliminating the reaction component of expanding gases escaping the muzzle in a forward direction.The original use of the recoil booster was to provide additional energy to move the large barrel/bolt mass on recoil operated machine guns.The primary German machine gun of WWI was the Maxim-based MG 08, a water-cooled heavy machine gun which used a muzzle booster to increase rate of fire and reliability.