I only bump the shoulder back about .002 as well. But I definitely have had carbon rings. I went about 100 rounds without cleaning at a shooting school with a 28N and I was getting sharper recoil and less drop than expected. I stopped shooting and researched when I got home.
I bought one of those piece of crap travel 4 piece cleaning rods and only use the final piece. It put that in a cordless drill and put an oversized nylon brush, one caliber larger, in the end and wrap a patch around that. I use iosso and use that for maybe 10-20 seconds at a time, stop and clean it out to see progress, and go a few more times if needed.
If your experience is like mine, you may see a noticeable drop in velocity from your most recent shots. Depends on how much carbon was baked on.
I have a 6.5 PRC and have cleaned the neck like that once so far in about 150 shots. It isn't as overbore as a 28N but it is a hot enough cartridge that it 100+ shots could easily cause a carbon ring.
