1. Have a gun on your back, for example SKS.
2. Pick up another gun such as a Mosin in your hands by looking at it and pushing F (by default).
3. Log out (optionally standing next to another person watching so they can see the procedure if you are one of the devs)
4. Log back into the same server. After the wait the Mosin will be on the ground, but it will also still be in your inventory, including whatever ammo was loaded.