The MP5 had been in my inventory. The MP5 was in my hands and as well it had a place in my backpack.

The buttstock had been in a military-tent-inventory. The Inventory of the tent was quite full.

Instantly after dragging the AK-Buttstock onto the the MP5 it vanished within the inventory of my backpack but I still had it within my hands.
I still was able to shoot. The amount of amunition within the 30-mag decreased while shooting. I was able to drop all the attachments (handgard, flashlight, compensator, 30 mag) to my inventory by rightclicking on the item (mp5) in my hands. I was not able to drop the gun off my hands. I was not able to drag and drop anything other to my hands to switch this other item with the gun.

While still having the gun glued to my hands the buttstock stayed within my backpack at the place where the mp5-k originally had been.

When I logged out the weapon was gone. I do not think the gun glitched to some lower item-slot within the military-tent (where i cannot scroll to).

Server was good, only me in the server, persistant and had a (full) reset earlier (and restart / no loot explosions yet).
stable, NOT experimental