The culprit is potassium carbonate, a product of combustion. At high temperatures the potassium carbonate fuses to form a hard crud ring. Fusing of the potassium carbonate is caused by high environmental temperatures, large powder charges, hot primers, lubricants in the bore, some patch lubes, some conical lubes, and a failure to let the gun cool between shots; among other things.

Triple Seven is a hot burning powder. The gun heats up very fast, especially with large powder charges.

Recently i've started experimenting with my supply of Triple Seven powder. My .54 New Englander rifle loves 70 grains of Triple Seven and a patched round ball. It fires every time using a #11 Winchester magnum cap.

i'm also shooting 70-80 grains of Triple Seven and sabots in my .50 CVA StagHorn. The cap is the Winchester magnum #11.

i don't like to swab between shots, so generally i don't do that. The StagHorn has a pristine bore, i've fired as many as ten shots of Triple Seven from that gun without swabbing. Can't do that with the New Englander, maybe because the bore has some rust on the trailing edges of the rifling.

One day i tried Triple Seven in my .54 TC FireHawk. Forgot to swab the Militec lube from the bore before firing. In addition to the crud ring, the entire bore was coated with a very hard black crud. Removal of that crud required Windex with vinegar on a Scotch Brite patch. Took 20 minutes to get the bore clean.