I just got off the phone with a helpful and polite gentleman at US Dept of Commerce. Here's the query I sent to them using the form on their website:
"I am investigating the legal requirements for exporting EAR99 categorized shotgun components from the US to Canada by a Canadian Citizen.
I am interested in stocks, minor components (springs, pins, misc parts, etc.), and scope mounts, bases, rings and rails. I understand from your web page
http://www.bis.doc.gov/licensing/exportsoffirearms.htm that these components fall under EAR99. What I cannot confirm from your website is: Can a Canadian (or an American for that matter) export these items from the US to Canada without any US export paperwork (i.e. export permit or export license)?
Your FAQ page suggests this is the case (i.e. as a Canadian I can travel to the US and purchase these items and return to Canada without further export documentation) but I would like to confirm if this is so."
Over the phone, in response I was told that yes all these items would "likely be deemed EAR99 items" with no restrictions for their export from the US. I asked specifically if a Canadian citizen could travel to the US and bring them back or mail them to myself in Canada and he said yes. To be extra safe he suggested bringing along excerpts from their documentation regarding EAR99 or if mailing/shipping, include a letter to that effect in the package.
He was also clear that the stocks, pins, springs, etc. refers to sporting shotgun parts only. Rifle/handgun parts are governed under ITAR as applied by US DOS. He was also clear that shotgun ammunition, including components, is all restricted for export and needs DOC paperwork. Lastly, I/he were clear that the scope mounts, rings, rails, etc. governs for rifle, handgun and shotgun - these are all regulated by DOC and these parts are considered EAR99 and as long as there are no optics involved (i.e. not the actual scope), the mounts are EAR99 and are free to be exported.
He was also careful to point out that it is up to the receiving individual to ensure the parts are legal here in Canada as he recognizes exporting from the US is a separate process to importing into Canada.
Nice guy and for the price of a long distance phone call, I'm content to bring shotgun bits back into Canada. At least some shotgun bits. So a win here.
Check the DOC docs though because some specific things you might not think like complete trigger groups and specifically shotgun magazine extensions are indeed regulated for export.