Ah Firearms-different culture all together up here....the regulations here are quite strict and are by in large directed primarily at handguns. For instance....in our larger airports there are US Pre-clearance Facilities with USCBP officers employed there. They are not allowed to be armed in this country - period. We have to station Canadian Police officers in those faclities in order to ensure that there is an armed presence. It is highly unlikely, more likely impossible, that a US visitor would be allowed to carry one in Canada. The sheer volume of paperwork involved would discourage even the most ardent gun fan. Can you sneak one past the Cdn Customs -
probably but if you get caught either at the crossing or in-country with it you're in big trouble.
As LeoB correctly pointed out you can get permits for rifles and shotguns - during hunting season. But NAP1974's advice is best followed.