Thanks, Dan, for clarifying that $100 limit.
I just heard from my US Customs contact.
A modern rifle barrel from Canada coming to the US needs an import permit! I found a like-new Ruger .45-70 barrel on EBay, and expressed interest. He directed me to ATF and I called Washington DC (after three other contacts) and was directed to a "Form 6, Part 1". It is available online at the ATF.gov website. Find form #5330.38
Processing and approval takes four to six weeks! No fee.
SHEEESH! Bringing gun parts home is nothing like the antique rifles I've brought home on business trips anymore!