In my limited experiences, it has to do with the burn characteristics of the 777. Both my Knight Wolverine and my Encore show the crud ring. It is not as bad in the Knight, but it is there. This Knight has shot pyrodex pellets for a few years prior to 777 and never had this problem. That is with #11 caps as an ignition source.
My Encore came with a concave breech plug. I liked the idea of the .25 acp conversion so I ordered it before my barrel arrived. My barrel came in first so I shot it 6 times with the 209 primers. Spit patch between shots allowed proper seating of Dead Center Sabots, but crud ring was prevalent. On one stroke, I thought I would never get the patch out.
The new breech plug is here and will get tested this weekend.
Smoky