Not true on that. Stolen items remain the property of the last legal owner; anyone who buys stolen goods, even in good faith, must surrender them for return to the rightful owner when the true situation is learned.
As to the present thread, I think it is very unlikely that the OP will ever get the gun back. One problem might be whether he can prove the gun ever belonged to him or his father. Knowing the serial number is not enough, and there is evidently no official record of his or his father's ownership.