he inspected the gun on 12/7 and said everything was fine.

send him a message that says if you don't hear from him by friday (12/17, one week after he told you it was the wrong gun) that you will consider the matter closed and will no longer pay to reverse the trade.

after that date if he is still adamant about reversing the trade he can send the gun back at his own expense, but after 1/1/2011 you will consider the trade finalized and accepted by him and if he is still upset he can go dig wiener size gopher holes in a golf course sand trap of his choosing.