It may be that the barrel was "pickled". This is basically boiling bleach and water outside so you don't die from the fumes, and then, after removing the bluing (super simple to do), lowering the barrel (with all holes plugged) into this boiling tray for a few minutes. Then reverse its direction in the tray so that it rusts evenly. Did this with the barrel on my Traditions Pennsylvania longrifle back in about 2004. Gunmaker that I did this with learned it from Hershel House.
The barrel instantaneously turned red with rust. Part of the trick was to get these tiny balls of rust equally concentrated along the entire barrel. He had to fabricate a tray and a burner system to heat the 50% solution of bleach evenly to get those little balls of rust. Absolutely looked like a ruined barrel when we took it out. Let it cool off a bit and then took it over to his grinder and used a wire wheel to knock off the rust balls. We didn't take all the way back to bright, but close to that. Then, once we had the finish where we wanted it, he just oiled it....Done deal. I used it at a LOT of reenactments until I bought my Early Lancaster rifle from "tg" here on the forum.
Here's a look at the pickling on the barrel of my Traditions longrifle. Object was to make it look like it has had at least 20-years of hard use.
If the bleach mixture doesn't boil evenly, you'll get big divit and small divits. Part of the trick is designing the burner to be able to hold a longrifle barrel and to heat it evenly all the way along the tray. Otherwise it will turn out blotchy. I haven't used this rifle in many years and the barrel has gotten a little darker than the last time I used it.
