Depends on how you are "importing" them (in my experience).
The issue is not "importing into Canada" but "EXPORTING them out of the US. Unless the US shipper has the appropriate US EXPORT permit, they place themselves in SERIOUS JEOPARDY. Wonder why Midway doesn't ship to Canada? They got caught up in this mess.
Yes small gun part stuff sent via mail can come in "under the radar" as "small machine parts" BUT a few years ago at Coutts AB , when the US CUSTOMS were stopping OUTBOUND traffic prior going through CANADIAN customs and enquiring "what are you bringing back", I declared a HANDFUL of small OLD USED GUN PARTS for a Win 1886 and a new stock set for the same.
After 3 hours of "diligent research" by the Custom Agents (who were very congenial, objective and inoffensive) they concluded I could keep and take home " the furniture" (their exact words) but not the handful of small internal parts. As they explained "anything IN, ON or THROUGH a firearm " could be EXPORTED only under permit relating to the UN IATT (International Arms Trade Treaty) - a permit the US EXPORTER must obtain and which US Border Services REQUIRES. Even such small parts are considered a "munition of war".
Canadian Customs couldn't care less about about the gun parts providing you declare them as part of your purchases in the US.