Legal, if the sender has a US export permit, and you have an applicable import permit.
Certain parts are restricted for export by the United States Government.
Barrels, barrel blanks, bolt assembly and cylinders, receivers, frames, bolts, and high capacity magazines-magazines over 10 rounds but below 20 may require an import license for the country of destination. Magazines 20 rounds and larger are only for military, government and police agencies and require an end user certificate.
I certainly would'nt want to get anyone Stateside on the bad side of the Homeland Security people.
Not to mention you yourself as the person the package was being sent to would most likely get yourself on some sort of watch list as well.
Here's a link
www.dfait-maeci.gc.ca/eic...q-en.asp#4
Best think of buying something here in Canada.