A machine gun( 1 )is provided which has a detachable barrel ( 10 ). The barrel ( 10 ) has a plurality of axially extending outer barrel ridges ( 20 ) integral with and projecting from the top of the barrel ( 10 ) near its rear. The body ( 3 ) of the machine gun ( 1 ) has a barrel catch which is integral with a barrel holding socket ( 31 ) and having a chamber ( 33 ) adapted to accommodate the barrel ridges ( 34 ) reciprocal between locking and unlocking positions. The locking pin ( 34 ) has a plurality of downward projecting, axially extending body ridges ( 35 ) which together with a stop means ( 36 ) snugly accommodate the barrel ridges when in the locked position, but intermesh with the barrel ridges ( 20 )in the unlocked position.