I've found using a 10 ga brush vs a 12 gauge gives you more friction. Those streaks need some work but I bet they'll come out. I tried a bit of naphtha/ white gas on those and it worked good with the 10 ga brush.
Years ago I bought a long brass gun cleaning rod from Brownells I chuck into a cordless drill. I also sometimes do what you do which is to go a size larger on the wire brush.




















































