If you have a weapon and an axe on your back and a submachine gun in your inventory do the following:

my assignment for hotkeys: 1= axe, 2= SMG, 3= Rifle

• Take the main weapon in your hand (press 3)
• Switch to your smg (someone jumped you or whatever) (press 2)
• Switch back to your main weapon (seems like you suvived and want to keep sniping) (press 3)
• Switch to the axe or anything in your inventory or just put the weapon away (zombie attack? bleeding? morphine? need to run?) (press 1 or 3 or take item in hands)

the character will now drop the main rifle to the floor since he put the SMG on his back earlier. If you do this while running from a zombie or anything else its most likely "good bye precious rifle" because you will most likely not realize what happened instantly, especially in 1pp (atleast in my cases, happened 4 times now in a week)

My suggestion would be that if you put the SMG back the game first checks for inventory space before putting it on your back.

I know i will get used to this and be fine but if it happens to me that often im pretty sure many many other people experience this too which truly is kinda frustrating since you most likely have a pretty valuable rifle when you reach the point where you carry both