If you click on the link in my previous post it will take you to the detailed rules.
In short, you can get a 60 day temporary firearms licence at the border/airport when you arrive. You don't have to be a Canadian to get the regular Canadian 5 year licence, but it's probably not worth the bother for someone who isn't going in and out of the country frequently.
An airgun that launches bowling balls at 499fps wouldn't be a firearm under our laws. The 500fps limit appears to have no logical origin - just a nice round number when it was introduced 50ish years ago. The authorities added in an energy element about 10 years ago when they realized that the new hyper-velocity plastic sabot pellets (which typically went 40% - 100% faster than traditional lead ones) were going to turn about a million low-powered airguns into firearms requiring licenced owners and/or registration.