Hi,
I had an experimental kpist and picked up a muzzle for an SMG, added the attachment and the entire gun vanished!
The location I lost the experimental kpist was the Klacksundsfyren lighthouse, I picked up an attachment from the weapons box in the small hut behind the lighthouse, A SMG muzzle I think, added it to the Kpist as an attachment and when I went to use the gun it was gone.