I had a very similar problem with my 308 Win when I tried using CFE223 powder.
I had bowed out of this thread bc the answer is pretty dang clear to me. I agree that the powder makes a huge difference. My carbon rings show up with IMR 4451 quicker than they do with IMR 4350 and a H4350 takes the longest to show up. Not sure what flavor they're using in the hornady factory PRC stuff, but 90 rounds and a carbon ring like that is no surprise to meOnly recourse in the long run is to use cleaner burning powder.