Mike, it has been a few years since I did but but off the top of my head this morning, you first must be able to demonstrate the purpose for having the gun in the US. US state hunting license or registration confirmation letter from shoot.
Then you have to complete a form from ATF that specifically lists by serial number the guns you want to bring in. Submit that, with a copy of your hunting license, to the ATF. They will take some time processing the application because they are normally quite busy overseeing Fast and Furious. (LOL, just my own bit of early morning bureaucracy humour.
Once they mail the approved application back, you are good to go. The authorizations last 6 months and you can buy whatever ammo you need in the US. You may NOT bring excess ammo back to Canada. Doing that is a whole 'nuther adventure in paperwork.
