Triple Seven burns hotter than the other subs. The crud ring is made worse by large powder charges and sometimes by 209 shotgun primers. The crud ring happens when the primary residue from firing, potassium carbonate, gets hot enough to melt and fuse.
My TC Black Diamond rifle uses #11 caps. There is no crud ring with 80 grains of 777 and the Winchester magnum caps.
Dutch Bill is the foremost authority on things blackpowder and blackpowder subs: He advised the industry for decades:
The hotter burning 777 can present a problem when you start pushing the charges above a certain point. It begins to form what the ml boys were calling a crud ring just ahead of where the patched ball or slug sat on the charge. Another deal where the gas temperature gets high enough to melt and fuse the potassium carbonate created by powder combustion.
So as you begin to increase the proportion of 777 to bp watch for any hard fouling to form.
Under the right conditions blackpowder and some other subs can leave a crud ring, although not as pronounced as 777.
Removing a 777 crud ring is not a big deal. Immediately after firing the last shot i swab the bore with Windex with vinegar. At home i swab the bore again with Windex with vinegar followed by a brass brush on the ramrod. In about three minutes the crud ring is gone.