I've just had a couple of times where the enemy was full screen <5yds and i sight and open fire half clip and they didnt die, on one occasion the other guy had a pistol. It seemed weird as they didnt even die afterwards and they would have been too damaged to bandage. When i respawned I fired at a wall and some wood and there were no bullet holes or debris effects so it seemed as though although it was playing the animation nothing was actually "firing" from the weapon. Tried with a different weapon and it was the same so re-booted.