Canadians who enter the U.S. do not require any visa and are allowed to stay up to 180 days if they are not seeking employment or attending school. Only need a passport since June 1st/09.
Without an immigration type visa, then, I would say that you would have to be a continuous resident in a state for 90 days and possess a hunting license to fall under the y(2) exception of 18 USC 922 in order to obtain a firearm in the US.
That's just the way I read it, though. I am not a lawyer, and I am not that familiar with immigration laws.