There's a lot of uninformed advice in this thread - Wendell posted the relevant link from Global Affairs Canada a few comments up that supports my statements on the first page. Again, you're exempted from requiring an import licence as long as the total value of all parts is below $100, and you can't import any "significant parts which includes; barrels, slides, cylinders, bolts, frames and receivers". It goes on to also state that shotgun parts and ammo/ammo components have no exemption.