1. Have a rifle hanging on your back (I also had a Dry Backpack on - don't know if that affects the results)
2. Find another rifle (obviously) and take it from the floor into your hands
3. Drop your primary rifle on the floor (don't let go of the 2nd rifle)
4. Click the red cross to unequip the 2nd rifle from your hands
//The rifle should either immediately appear below your backpack (in the inventory), or appear in the usual rifle slot, and then move down under the backpack
5. Equip your old primary rifle directly from the floor (don't take it in hands)