I'd walk away. You now know he is no friend but it simply isn't worth getting into trouble over it. If I owed money to a friend, I'd be giving money back before even thinking of going out on the town.

You could make some sort of attempt to get a payment plan out of him and if he won't write him off as a lowlife scumbag mooch. Legal attempts to get it back are going to cost you money, time and aggravation. Other methods can end badly.

If the paperwork for the gun is in his name, sorry.

Steve