Ken: Same in Canada. Barrels are not in the definition of a firearm
see here
Canada Post will ship complete firearms in Canada
Firearms cannot be shipped via air and cannot have any markings on the outside of the packaging. The Customer is solely responsible for meeting all Canadian Firearms Centre regulations.
Since barrels are not a firearm according to the CFC, there is no issue.
Here are
Canada Post rules
So going South, neither country considers barrels to be a firearm. It becomes "ordinary" mail. But, the outside packaging must have the declaration of contents form attached when moving from Canada to US. Some folks prefer creative descriptions "gauged tubular steel" others prefer less creative "barrels" or "barrel set".
Canada Post passes packages to US Postal Service which has a good reputation among US folks on this forum. See
here
I have only brought them north with frames and stocks included
Cheers,
Jack