Nice work, Bill - you are really catching on to this gun building thing! Actually, a lot better than some of the work I've seen over the years.
Couple things: Carding can also be done with a wire wheel from supplier like Brownell's. They have a couple great wire wheels, one with .0025 SS "bristles" that are actually very, very soft. I've hit my fingers several times - no damage. The bristles will not damage the colored metal, they just brush off the rust. They are about $25. For hand carding, a Carding brush they have does a great job. Oh, the wheel has a 1/4" center hole, so you can drill it to fit your arbor.
Also, if the builder wants a blued gun, rather than a browned one, the only thing needed is boiling water for a few minutes - changes the valence of the iron oxide, which in turn changes the color. That is how rust bluing is done, just boiling water immersion. Small pieces are easy, do it on the stove top. Barrels really need a small trough with a couple burners to heat the water.
That is going to be a right nifty firearm when you are done -raffle it off and you might make some money. Or not.....