Triple seven is not the only powder to create the crud ring. My son's stainless Apex gets a crud ring (not quite as bad as 777) using Pyro P after only one shot. We have to use a wet, not damp, patch to swab between shots using that powder otherwise the range rod gets stuck near the BP. I've also had crud rings in various guns using Black Mag 3. I never have determined the exact reason some of my guns have CR problems and others don't but my guess is, as stated before, it comes down at least partially to BP design.