Guess this is the problem shooting cheaper lead cast bullets. If you don’t get the recipe right (or don’t have time to get it right), this will happen to your barrel.
It’s called Leading. Where hot gasses cut through the lube/lead and leave lead deposits on your barrel. As leading increases, accuracy will be affected and pressure in the barrel will increase.
It’s not permanent. It’ll come off with a good Hoppes scrub and that’s what I just did. Guess I’m going back to shooting my Moly bullets. Not only does Lead Cast bullets “lead”, it’s also VERY smoky because the lube burns up and generates a ton of smoke.