There is an opinion that the wording of the CCC prohibits cutting down even a restricted rifle to a shorter than 18.5 inch length. (As with many aspects of our gun law, opinions vary)
One possible option that might satisfy both sides of the border, would be to have Robarms counterbore the factory 16 inch barrels to the desired length. My reading suggests the barrel would be a legal 16 inch barrel as far as the US State department is concerned, and as far as Canada is concerned, it would be whatever the length is from the breechface to the end of the rifled section. Cutting off the useless counterbored section after import might be kosher.