From my experience when I've been in combat with people and zombies my gun works perfectly but if I was to leave and come back 10 minutes trying shoot at zombies ( how I test if the gun is still working) it will fire but nothing well happen and I go as far as to walk up to the zombie and let 30 bullets go and nothing will happen and it's the same with players won't get hit cause there's nothing coming out of the gun despite the fact your gun is making a good show of it
The most common gun that it happens to is the AK but as far as I know it happen to all the guns more or less