When you carry and handle a Mosin it will bug randomly. The bug is that you can't take your Mosin out of your hands or take it in your hands. The only way to take it out of the hands is by forcing it (taking another item in the hands, doing a gesture, etc.). It will then go on the back or on the ground if you have another weapon. The only way to get it back in your hands is loading it with ammo. Even after that, you can't take it out of your hands. Now relogging helps with most of these cases. However in this case it just vanishes your Mosin into thin air and at that point you're gunless.