a common bug in .63 exp right now is the one where people find MP5s (mostly) with a mag in them and are unable to unload the mag and when they put the gun in their hands they can no longer press F or remove the item from hands.
I found a way it happens
if you have the magazine rifle (MP5, Skorpian, AK) on your back, and you have no space in inventory, if you put a magazine in your hands and press a hotkey to pull out the rifle, the mag will go into the gun because there is no more space, and this causes the bug.