If I recall the US Postal Service instructions for shipping firearms (long guns), you must NOT tell the clerk what's in the box. You can also insure for very big bucks if you register the parcel, something I strongly recommend. I had a curious clerk ask what I was shipping that was so valuable and I used Stan's line, "precision machined parts."
As barrels are not firearms (actions are in the USA -- in South Africa barrels are the key element), you should be able to ship with any carrier.
Regards, Tim
