Some stuff I've tried:
The gun's condition is pristine.
The bullets are pristine.
The guns are in fact loaded and chambered.
I have verified game cache, it said there was corrupted files and they were re-downloaded.
I went out and bought a new mouse. (needed one anyways)
Here's some strange stuff I've noticed:
It does not make a click sound when trying to fire.
It sometimes happens in the middle of a magazine. Specifically my scorpion.
Shotguns are ridiculously affected, but rarely when I'm not in combat. I've stopped trying to shoot people with them. I don't think I've had trouble shooting anything except when aiming at a player (can shoot zombies, animals, random shit just fine as far as I can tell)