Sounds like the rifle is a good one. As for the crud ring, well it seems to happen when shooting Triple Se7en powder. You could go the the FFFg grade of Triple Se7en and see if that helps. The Winchester W209 is a good primer. I would not change. Although there are other primer options out there.

Also I can not state enough the importance of good swabbing. You mentioned you ran a spit patch down and back. Well next time take some 50/50 alcohol and windex (car windshield washer fluid) and use that to swab. Run two damp patches down the barrel and then a couple of dry ones.

Do not use any bore butter or lube on the plastic sabots. It can cause them to slip the rifling causing uneven spin from shot to shot and therefore the accuracy would suffer. In fact keep the bore butter out of the barrel (this is my personal opinion). After you dump your powder, I put the sabot in my mouth and get it just a little damp. Then push it past the QLA with a short starter. Then use the long ramrod to seat the projectile in one solid push. Whether the moisture from my mouth really helps that much, who knows... makes me feel better, and I do it every time, so I will keep on doing it.

Hope your arm gets better and you find more time to shoot.....