Well, I don't know what the hell the spots are, but whatever they are are capable of resisting bronze bristles saturated with Barricade, Ballistol, WD-40 and even Hoppes #9.
I would saturate a patch with the above (not all at once), let it sit on the spot for a few minutes, then brush at it with my bronze brush dunked in the same. Never got anything off, apparently, even though when I would swab the bore with dry patches afterwards, I would drag out black on the patch.
I oiled the entire bore with Barricade, then saturated a patch with more Barricade and snugged it onto the spot with my spare jag. I'll let it sit overnight and look into getting some Evaporust tomorrow.
I've had surface rust form like that in my smoothbore. I don't think you can ever truly eliminate the possibility of it happening. Just keep the barrel old when stored and it will be fine. That minute an amount of surface rust won't hurt anything.
My main concern is how it formed in the first place. I've been reoiling the barrel once a week pretty religiously, so it is making me doubt that routine, as well as the possibility of more surface rust like that up in the bore where I can't see it.
That, and the fact that it isn't coming off no matter what I do to it. Brushes, oils, nada. Makes me wonder just what it is, because if it was rust I would be expecting to pick up streaks on patches, and I am getting
nothing. Nice, clean oiled patches, up and down the bore, nice bright shiny bore afterwards, with no dark spots (at least that I can see holding my eye and a flashlight to the bore. I don't own a borescope)
Yup. Since I live right on the coast in New England, I am concerned about salt-water air, and have been applying Barricade to the bore about once a week for a month now. No issues whatsoever, until I bent over to insert the jag into the mouth of the bore today and noticed those spots. They are nowhere else in the barrel (if the top of the gun, where the sight is, is North, this spot of spots would be West, opposite the lock on the East), and it looks like they only extend down the bore an inch or so. I'm not feeling any roughness when I send a patch down the bore, wet or dry, so if it is rust it isn't very extensive.