Originally Posted by Drache
The parts section is restricted to stop underage people from buying parts and building an entire gun from scratch (which has happened).
Correct. The parts section was open to all for a long time, but too many minors were buying parts and building guns from scratch.
As for parts in Canada, other than receivers, there's a cornucopia of replacement and upgrade parts available, be it for internals or externals, that are available without the requirement of being 18. With your M4, there are probably more Armalite parts available than for most other types of guns combined. And you don't need to get into the ASC classifieds to get them.