When using a Mosin while a loaded Longhorn is in the inventory and option to reload the Mosin is constantly in the mouse wheel scrolly list. If it is selected, any ammunition in the Mosin vanishes and is replaced the single shot from the Longhorn.
This is a fairly major problem as it can cause large amounts of ammunition to be quickly lost.
Have a Mosin (or sawn-off variant) in your hands and a loaded Longhorn in your inventory.
Stahp Eeeeet!
Actually it's not true. You reload your mosin with 5 shots.
If you empty the mosin, you can't combine the ONE shot that you see in the inventory with other mosin Ammo. It say's that the ammo is already full.
Rightclick the ONE Mosinshot slot and Empty, then 5 Ammos appear again.
For a reason I don't know there seems to be an invisible mag for Mosin, you can load it once with 5 shots. Unload it does only one shot, but in fact its a mag.
But yes the Longhorn and the mosin wants always reload each other, which is pretty annoying^^
Tested on 0.48 with a Mosin, Short-Mosin, Longhorn, all three in the inventory