How did your FIL get the idea that you should buy the gun, no matter what it is worth? Unless you slipped up and told him you would buy it, you have no obligation to do so, period. If you want some amusement, tell your FIL that the gun is worth a million dollars but you don't have that kind of money and he should take it to a gun show. They'll laugh him out of the place and you will get to enjoy his ranting.
P.S. Divorce may follow.