From the US perspective this is not a problem. Remember that Canada is not the US though ... often casually forgotten by many Americans due to the similarity in the language and the ability to just drive there : )
I have done this myself from the UK. However in the UK the barrels must be handled via a RFD just like a gun with similar exporting permitting requirements. Given the historical ties of the Canadian legal system to the UK there is a good chance that Canada has similar requirements so I'd check with your Canadian source on that. You might hit a stumbling block there.
Once they're on the truck/plane headed for the US you shouldn't have any difficulty.
Jeremy
