Semi auto fire plays the 'full auto' sound clip per shot, the one where it plays one shot, followed by the start of another but the gun only shoots once.
Full auto fire plays the semi auto sound shot.
Would really like the second shot part to be fixed; especially if you want to semi-auto while in full auto. Then it wouldn't matter which 'sound file' is played while shooting.
Edit: Just realized it's based on length of mouse click. Though it still shouldn't be playing the 'second shot'