I have seen this too and thought it was lag at first. But sometimes you can shoot and kill and alien with one armor piercing bullet, but reduce the accuracy. Not sure how that works.... It isn't game-breaking, so I don't mind too much at the moment.
The accuracy always seems bugged to me, if you use a fully auto weapon you will never achieve it no matter how well you aim.. I think its lag but the hits don't seem to register properly