Casting would just be the first step. After that, the metal would be forged into a billet, which would then be machined into a barrel. That's not the same as "hammer forging", which involves taking a metal tube, putting it over a mandrel, and using a special machine to form the barrel by compressing it over the mandrel. I'm probably missing a good bit of detail here, but that's what I know about making barrels. I believe hammer forging wasn't used to make barrels until the 1930's.