Big Blue, the problem with the 45-70 particularly is that we don't drive the pressures to the level where rifle powders are going to burn cleanly. It takes about 40,000 psi to get rifle powders to burn cleanly, most of us are shooting loads making 28,000 (Marlin 1895/Win 1886 class) or less.
I'm willing to bet that most of the loads folks are using in their NEF don't exceed Trapdoor levels which are 21,000 psi, half what is required for a "clean" burn.
I suspect there will nearly always be enough blowby gases to drive out any debris from the previous round and if not, the bullet certainly scrapes the bore down, I have not ever noticed an accumulation of powder debris in the barrel. I have heard and read disscussions of such debris being "ironed" into the microscopic pores of a barrel, and while that might be true, I wonder if that happens before or after minute copper or lead particulate begin to fill those voids, or if any of that really occurs at all. I don't have an electron microscope, but I do have a caliper that reads to .001" and I have yet to be able to ascribeany accuracy degrdation I can measure to bits of material being ironed into my bore, regardless of what those particulate bits may be. It the end, those disscussions begin to sound like people picking out the flyspecks from the horsesh#t.
I would love to see a study on the relationship of accuracy degredation and cleaning. With the electro/chemical cleaning processes available, it would be easy to document a rifles accuracy in respect to the cleanliness of it's barrel.
But I begin to get off topic.