I'm not sure about exact numbers, because I can't find the thread I mean, but how it works (or is supposed to work) is following:
Player can steal kill from another player, just like before, but NPC can't steal your kill by "just the last shot" - if the target dies within certain time (10s?) after your last hit, you still get credited. So it's important to land a hit from time to time, even if you let cops do your work.