You can quickly reload a Mosin in your hands by having a loaded Mosin in your back. Might happen with other weapons as well.
- Legacy ID
Add 5 rounds to Mosin in your back.
Take another Mosin into your hands.
A clip/mag reload icon appears now and by pushing 'R' for Reload
Rounds from the Mosin on your back go to the one in your hands
I suspect the loaded Mosin on your back, the game thinks of it as a clip.