I took a shotgun off of a dead guy that my friend killed. When we got into a firefight I realized that I couldn't aim down my sights or even hip fire. However my axe and all my other items worked fine. This was fixed after I relogged.
Later I killed some bandit and took his Mosin. As I was leaving the airfield I realized that I was being followed. I hid in a bush and I had the perfect shot on this guy. He was oblivious and lost my position. But, alas, I could not fire the gun or aim down my sights.
I've tried dropping the gun and picking it back up but the only thing that seems to work is relogging. I've posted about this on Reddit and many others are having the same issues.