Right before the raid one of the cops works the bolt on a machine pistol. He works like you would on a semiautomatic pistol. Back then let it go forward. Submachine guns fire from an open bolt. The means he have pulled it back, it would have locked back. Waiting for the trigger to be pulled. The bolt going forward fired the weapon, as long as the trigger is back the bolt keeps blowing back, then forward to fire again.