RoostRider: "I would never put a painting this valuable on my wall ..."
Actually the wall is the safest and best place to store valuable art, so long as it is out of direct sunlight and not in the playroom or smoking room, and properly anchored. Art in the closet gets crunched, art on the floor gets kicked or spilled on, art in the cellar gets moldy or flooded out, art in the attic gets dry and cracked.
Back to guns, I'd tell the guy the gun is worth more than he owes, and that while I'd happily take it to settle the debt, he'd be doing me a big favor by making that trade. If he was a tenant, I'd offer to give him a few more months free, particularly if it looks like he might need them. Whatever deal you have to make, don't let him take it back; he'll go and sell it to a pawnbroker or gunshop for a little more than he owes you, in order to pay you cash. Get that gun. But don't become a swindler in the process. Let him be doing you a favor, and repay the favor as and when you best can. There are many, many ways to repay a favor like that besides cash.