This bug appeared when I dropped my Mosin on the ground for a while and then picked it up. One minute later a friend caught up to me and said there was a Mosin on the ground even though I had it on my back. He picked it up which effectively swappinb the Mosin on my back with an M4 (that's the weapon my friend was carrying when he picked up the Mosin). There were two other people around and none of them have seen the Mosin on the ground after I had picked it up.