So I was looting and had found and SKS that was on my back. I had so far been able to successfully pick up several other weapons (in the hand slot) and then drop them on the ground without them disappearing.

I found a MP133 pistol grip and wanted to move it so I put it in my hands

After placing it in my hands I could not place it on the ground, nor could I put anything else in my hands. I checked to see if inventory was otherwise responsive and it was (could place objects on ground, open cans, ect.)

I tried climbing a ladder to displace it. This put the shotgun into my inventory though I could now not place it in my hands, I could now switch between the SKS and my sidearm with the shotgun still in my inventory.

I re-logged and things got weirder. Now the shotgun was on my back and the SKS was still in my inventory. I could take the SKS out but it was now only in my hands. Upon placing the SKS on the ground it disappeared.

Shotgun the spontaneously disappeared as well.

Cheers