At first I wanted to write a correction to nstahl's post, but then I figured I would end up giving you bad information too, but then I decided that I had to be a know-it-all on the internet.
The import law is that you can import an unregulated firearm, which is a firearm that fires between 366FPS and 499FPS. Anything below that is a replicate firearm, and cannot be imported, and anything over that is a regulated firearm, and cannot be imported.
The CBSA will look for official documentation for your gun, either within the package, or online (they will ignore any notes the distributor added with the package to say it shoots the legal limit), and if they find that it is not legal, it will be destroyed. You will not be able to return the gun to the retailer. And unless the retailer has money to burn, you will not receive a refund for good seized at the border.
As for what happens if they don't find documentation, I'm not too clear, so don't take this as fact - but, from stories I've read, I guess that they'll likely request documentation from you, and if you can't provide any, it will be sent to purgatory for several years, waiting for the RCMP to chrono it. (the purgatory for years with the RCMP is fact, though)
Of course, you'll be notified if the package is seized at all.
All in all, it's risky business trying to import an unregulated firearm. Even if it shoots the legal limit, it could still be seized and sent to the RCMP for testing.