Customs has zero to do with it. This is controlled by the Department of foreign affairs and international tradeon the Canadian side.
Importing & Exporting into Canada
The rules have changed for exporting firearms parts into Canada. We must now get export authorization for shipping both barrels and scopes to Canada. With barrels we are required to get an export license from the U.S. State Department. In the past Canada was exempt from this requirement but we now must get the license as we do with all other foreign countries. With scopes we are also required to get export authorization but it is through the Department of Commerce.
Before we are able to apply for an export license the end user (the purchaser) must get import authorization from the Canadian government. Individual purchasers may get this authorization on their own or to avoid the paperwork you may place an order through one of the importers listed below. Here are the steps to get that paperwork:
The document is an International Import Certificate issued by;
Export Controls Division (EPE), Department of Foreign Affairs and International Trade
Export & Import Controls Bureau
P.O. Box 481, Station "A"
Ottawa ON K1N 9K6
1. Obtain the paperwork from above agency
2. Complete and return paperwork
3. DFAIT will issue import paperwork
4. Importer submits paperwork (originals or certified copies) to U.S. exporter