In my gun, the 777 crud ring only happens with pellets. I've had no problems when using loose powder, be it 777, or Pyrodex.
I was shocked last week. After cleaning my barrell thoroughly at home, I looked into the barrel and I could see a distinct ring at the place where the base of the sabot would be. The ring wasn't just crud, it was indented right into the barrel. This is in a TC Triumph that I bought new last Christmas, and has probably less than 50 shots through it.