Assume you are driving...you will have to register the gun at the border with the Canadians and pay a fee, seems like it was $50.00. you can use that copy of registration to go back into the USA, it proves that you had that paticular gun with you when you went into the country. Or to be safe you can go through all the paper work with US customs prior to entering, but I never had done so, as like I mentioned the canadian registration proves that you took it into their country from the states and didn't buy it in canada, all the US is looking for is to tax you if you bought something in Canada...