[thump_rrr]

Oh, ok.

By Federal law, the upper is not the controlled part. However, if you own both a rifle lower and a short-barreled upper, you would be in "constructive possession" of a short-barreled rifle.

The actual legalities of a Canadian citizen buying it, I have no idea.

I am pretty sure about the legalities of a Canadian purchasing it or complete (non NFA) firearms in the USA.

See section V for the summary.
https://www.federalregister.gov/arti...2001r-332p#h-8

Nonimmigrant aliens lawfully admitted to the United States without a visa, pursuant either to the Visa Waiver Program or other exemptions from visa requirements, will not be prohibited from shipping, transporting, receiving, or possessing firearms or ammunition, and the regulations will also no longer proscribe the sale or other disposition of firearms or ammunition to such nonimmigrant aliens.