IMO, crud rings come from patch lubes with ingredient that won’t tolerate high heat. I have shot many trail walks that have between twenty, and thirty, shooting stations without swabbing, using bear grease, or venison tallow, or a combination of the two. Most of the modern patch lubes have one or more ingredient in it that won’t tolerate high heat, and or pressure.
The other crud ring creator is a rough bore at the breech, from shooting very heavy charges, or phony black powder substitutes.
Hungry Horse