Some of the black powder bullet designs have generous lube capacities, and that can assist greatly in keeping fouling soft.
I haven't done a whole lot of BPCR shooting, but enough to learn that proper lube and its amount can be critical to extending performance length. Example (you KNEW I would do this)......Win 73 in 44/40 WCF, vintage 1897. W-W cases, CCI 350 primers, 36 grains of Goex 3F Flaming Dirt compressed about 1/16".
Bullet #1--Lee 200 grain round flat nose, sized .429", 30/1 Pb/Sn. 2 shallow, skinny lube grooves filled with SPG. This bullet has no shoulder, much like Lyman #427098. Accuracy went south at Round #4, Round #7 tumbled. Game over. The combined influences of GFD, very low humidity (10%-12% that day) and minimal lube capacity messed things up in short order.
I cleaned out the coal mine, and went to Bullet #2--SAECO #446, with far more generous lube capacity, same primer/case/load/lube/alloy. #446 also has something of a "scraper" shoulder on its leading edge. Accuracy stayed constant for 15 rounds, equivalent to that given in this same carbine with smokeless loads (1.25"-1.50" @ 50 yards). At Round #16, accuracy started to fall off just a bit, but i surely could have stayed in battle for a time with such loads. I finished with Round #20 well within the radius of the first 5-shot dispersions. My conclusions--with The Holy Black, lube quantity matters--and bullet design might play a role as well. I knew going in that SPG was good stuff for the purpose--Mr. Garbe knows a whole lot more about BP shooting and its nuances by accident than I'll ever know on purpose.
A round-about way of suggesting that if someone was inclined to try The Holy Black in their 44 Special or 44 Magnum rollerpistole, that a Lyman #429421 with its grease gallery filled up with SPG might not be a bad place to start. Those decadent lube capacities can have their uses, IOW--and a snowplow in a coal mine might not hurt, either.