OP, A lot of the information in your documents has to do with registration of firearms which is no longer valid. Only handguns require registration in Canada now.
I have been buying muzzleloading barrels from the USA since 1971, and have always referred to them as "machined steel tubing". I have never had an issue with Canada Customs or Canadian Border Services, and have received my orders in due course. I always have them shipped by USPS and Canada Post.