This has happened to me as well, with an AK-74 not a Mosin. In my case my butt stock vanished after painting the ak body with green spray paint, but when I dropped the AK-74 my friend was able to see the butt stock still attached and was able to pick it up and drop it on the ground, making it visible to me again. After this a really nasty bug occured where the AK-74 was duplicated and in my hands as well as my friends. It ended with my friend throwing the AK at the ground and it glitching through the ground and disappearing.