Changing the backpack with a sawed off shotgun in the backpack inventory while having it "in hands" caused the shotgun to disapear after switching the weapon for another main weapon (axe or mosin).

The sawed off shotgun was in Hands during the backpack changing process. Some minutes later while switching the weapon for the axe, the sawed-off disapeared. After searching the ground we found it in the old backpack which we left where we found the new one. Maybe the sawed off was still bound to the original backpack.