I believe Dr.Caddy, after the last rusting, boiling, etching, heated finished barrels and applied either motor oil and/or boiled linseed oil and let them sit overnight. The oil wiped off the next day gives a nice appearance and the boiled linseed oil hardens and doesn't look bad either.
These are Stub Twist from an 1899 L.C. smith hammer gun. I was going to use it as a parts gun because it had 13 small dents in the one barrel. After taking dents out, I refinished barrels and decided to keep gun as a shooter.

These barrels have boiled linseed oil and barrels were heated when applied.