I'm not completely sure, but this is my biggest lead on this issue. Your hands' slot will become de-synchronized on your client compared to the server.
On your client you might be holding a rifle, but on the server's end, you might just have your fists out.
This means that your bullets won't kill or do anything and just behave like ghost "bullets"
I don't know how to reproduce, but I hope you guys are aware.
https://www.twitch.tv/anton/clip/ArtisticGleamingSandpiperMVGame