Somewhere around here, on the THR blackpowder shooting board, are some links to some youtube videos that include cleaning a caplock shotgun. If you go to youtube, i'm sure you can find them; if you don't find something on here with the search engine.
Basically, what i do is put a little piece of leather under the hammer as a plug, and pour some (hot) soapy water down the barrel...then pour it out...repeat...maybe twice if it isnt looking cleaner on the next pass....and then take the bristle brush down there to loose things up, another little pour of water...pour it out...swab it out with a patch...then a patch with some bore solvent; this hopefully deactivates anything i didn't get with the brush and swab...and then unscrew the nipple, and clean from that end with a pipe cleaner. While the nipple is out, i run it under the faucet, and give it a dose of bore solvent. Be very careful not to cross the threads when re-installing the nipple! I imagine it would be expensive....and then i take one more pass over everything with some Butch's Bore-shine, and put her back in the closet.
This method might not be the best - i did it just as described and came back a few days later to find some new rust...so you might want to wait for some more informed opinions! Some of the guys on here know a lot more than i do.
Also, you mention bore-butter - i'm under the impression that bore butter is bullet-lube; not a cleaning product. A little bore butter on the patch of a round ball, or in the grooves on a minie, improves accuracy and decreases fouling. It is not a cleaning or metal preservation product, per say. (as far as I know!)
Stand by for some more informed opinions than my own.
Good luck, and good shooting. -Dd