But when I checked with a bore light, I noticed there were scratches down the bore against the rifling. Like a brush on a power drill was used to scrub the bore or something.
I suspect your diagnosis is right on target
Cayugad. That would be a sure way to make a barrel hang on to microscopic fouling. At a gun show years ago I overheard one of the dealers recommending the electric drill method to someone, and wondered how many people he suggested that to over the years - and how many actually did it.