The chaingun in the Doom games, like its predecessor in Wolfenstein 3D, normally fires an even number of bullets each time the the firing key or button is pressed. When only one bullet remains in the player's supply, that bullet is deducted and a single shot is fired, as expected — but as the engine does not check whether the ammunition has run out when producing the necessary pair of firing noises (which are DSPISTOL), the sound is still played twice as if two shots had been fired.