The best course for proof of prior ownership when bringing a gun back into the US is the form you get from a customs office. I do not remember the form number. That said the Customs and Border site specifically says ANY form of proof is good and that the form is NOT a requirement.
The only problem I ever had was in Mineapolis once when the customs agent was a black guy with a Jamaican accent and dreadlocks. I showed him my Canadian registration and he said he wouldn't accept a document issued by a Foriegn government. I told him I had a plane to catch and went on through without incident. I suspect he had a problem with my Southern accent.
The nearest customs office to me is the port city of Brunswick, GA over on the coast so I never have gotten around to picking up the customs form on any of my guns. I always take a copy of the bill of sale now though...Geo
