I reload for a lot of rifles using a lot of different powders. I don't clean very often but never had a problem with a carbon ring until the last couple of years, and solely with RL26. I have a custom 7saum and a RR prefix factory 700 270 and both are shut down right now with carbon rings thick enough that its hard to chamber a round. This has never happened on any of my many other rifles that have round counts much higher than these two. I've tried a few products to remove the rings and am trying CLR right now. I'm quite sure that "its the powder" in my case, as that is the only variable that's changed.
