Hi Matchlock,
Barrels were pinned using lugs dovetailed across the barrel or morticed into the bottom of the barrel. Morticing involved cutting a shallow mortice the shape of the lug in the barrel, then filing a dovetailed base on the lug the same size as the mortice. The lug was tapped into the mortice and the edges of the mortice were then peened down around the base to lock it in place.
In the 17th century barrels and locks were usually left white (polished), heat blued, or russeted. Heat bluing was usually just heating the barrel to a 525-550 degrees to impart a blue color. Charcoal bluing was also used. The book "Espingarda Perfeyta" has a chapter describing the heat bluing of 17th century Portuguese and Spanish barrels (the best in the world at that time). Russeting is a bit of a mystery. Some think it was some sort of brown painting or coating. John George in his book on English guns mentions it as a pigmented coating that was brown and had been used to protect metal since medieval times. Russeting was very common on English military guns and early fowlers. Gunsmiths in the 1630's charged 4 pence for whitening (polishing)and russeting a musket barrel.
dave