"I was at Canada Customs for three hours!" claims one exasperated shooter bringing back a few gun parts. Another, who can't resist bringing back some Chip McCormick mags for his 1911 .45 says, "I threw a copy of Guns and Ammo and Field and Stream on the front seat, and when they asked what I bought I just told them 'some gun magazines' and I haven't really lied to them."
If you know how to satisfy the Customs Inspector's needs, you can sail through with a minimum of extra effort. When I went to import some gun parts earlier this year, I called several customs people at various border crossings and sorted through their sometimes conflicting instructions until I discovered what they really need.
The epiphany came when an inspector at Sarnia's Blue Water bridge told me "We get into a lot of trouble with our bosses if we let something through that we shouldn't." She then explained that they did not get reprimanded if they took a reasonable (to them) amount of time to check if they should allow the goods to come in.
Thus, importing gun parts or magazines requires you to satisfy the inspector that:
the article is legal for you to import (i.e. the mag only holds ten rounds)
you own the firearm that it fits (to verify your claim it is for personal use)
the place of origin if the item should be duty exempt (made in the U.S.)
That means bring your P.A.L. or F.A.C. bring your gun registration with you for each gun you may be bringing parts for, obtain a "Certificate of Origin" if possible for goods manufactured in the U.S. and it would not hurt to bring along your Authorization(s) to Transport. Most importantly, bring the 1-800-731-4000 number for the Canadian Firearms Centre, and don't be shy about referring the customs officer to them for the answers to any firearms questions he or she may have.
If you know ahead of time what you will be bringing back, it doesn't hurt to call the port of entry you'll be using and ask what paperwork they would need. Write it down and get the name of the inspector you speak with.