Is this the start of some kind of corrosion? If not, what’s caused the ring marks and how can I get rid of them?
FOULING IN BARRELS
I have known plastic fouling show as ring marks, but it usually comes out with bore cleaning solvent and a bronze brush.
However, you should always be suspicious of ring-markings inside barrels.
They might be nothing more than obstinate muck or mild corrosion, but dangerous bulges can also show as rings.
I think a visit to a gunsmith is in order.