Presuming it was Oswald rifle, so as to answer the original poster, and with no evidence per se....
Guns were shown at the TSBD in the week prior.
Dougherty and Oswald get talking, and Dougherty offers to buy the gun.
Oswald thinks he can a great price out of Dougherty, given Dougherty is not the sharpest knife in the draw.
Thinking he will be getting some money from Dougherty for the gun, he leaves Marina most of his cash for the kids (whom he adores)
Oswald brings the gun in and leaves it with Dougherty to check out.
Oswald goes to lunch, Dougherty shots Kennedy with Oswald gun.