I agree with others that the problem you are having is something different than the crud ring. I have had this develop from using 777. The bore remains remarkably clean whereas the last inch in the breech develops a buildup. I have not had near the 6 inches of buildup that you experience. Only the last 1/2 inch of bullet seating is difficult with my experience. And even then I can get it to go with a little strong arm. Maybe your sabots are melting somewhat in the bore? Does that sound silly? Just a thought.

Also, many people get buildup from the residue of protectants like bore butter, Sheath, Rem oil etc. I am no expert by any means but I am under the impression that unless you are shooting patches/balls, you want a dry bore that is free of any lubes. Lubes catch all the byproducts of combustion like the snot in your nose catches dust. sabots are self lubing and powerbelts are undersized with only the self lubing base making contact. It is possible that you are getting a good puddle of oil in the breech from protecting it after cleaning. I run a mop with sheath down the barrel with the breech plug out. Then I run a couple of dry patches to soak up the excess. I then install the cleaned but oil free breech plug (stainless). When I go to shoot I run a patch or two of alcohol up and down to remove the thin film of Sheath in the bore. I end up with a very dry bore that shoots 8 to 10 loads of Powerbelts without swabbing. I don't know how sabots will react but I would think similar.