Answer to your question: ATF form # 6 or 6A.
Search on Alcohol Tobacco & Firearms website.
As stated on posts above, this process will take several months (4 to 6 months or more).
Once you acquire this form and present it to the border folks, they will require a VALID reason for you to have the firearm in the USA. Valid reasons include, possession of a valid hunting license from the state you will be hunting in, an invitation letter from a valid shooting event that you are registered to participate in, paperwork that shows you will be transiting the US for an event outside of the US, like hunting or shooting events outside the continental US.
A Canadian citizen possessing a firearm, of any type, for the soul purpose of "protection" while visiting the USA is NOT a valid reason and you will NOT be issued an ATF form # 6 or 6A.
This information is accurate and available on ATF's website.
Lakeside
