I cleared out 5 horizontal spaces in my hunter's backpack and it just would not go in.
I was not able to try putting it into another bag because I was holding the SKS, and when I dropped the shotgun on the ground, it disappeared.(Attempts to pick up a gun that you dropped while holding another one is also bugged and has happened many times). I tried to re-log in, because it is known that after doing so it should be there, but I lost track of what server I was in.