Tks, Richard- as to fit of ball- we always start with a ball that is .005" smaller than the bore. I've heard those 12 bores have shallow rifling and due to that, a .715" ball would be OK and still shoot well. Get too small and accuracy becomes a problem with heavy loads.

With the .715" ball, I'd use a standard 10 pound denim. That will mic about .020" to .0215", with Hoppe's 9PLUS for lube. Note the PLUS - it's a black powder solvent and lube. Regualar Hoppe's #9 isn't. some guys use somethig like a BP bullet lube, softened with an oil like neetsfoot or olive oil, melted in a double boiler and mixed than let cool. it then becomes a heavy grease lube with no evaporation. Good for loading blocks.

I forgot to mention sticking a couple patches or rag down the tube before grinding away. Thanks for picking up on that. It's easy to re-crown.
Another method I use if I want a very short cone to assist tight loading, is to use one of the tapered stone wheels meant for an electric drill, available in hardware stores. I don't know the angle- maybe 30 degresss per side? Anyway- wrapp the 320 emery and turn it with your fingers or if you can clamp the muzzle straight up in a bench vice - for faster results, you can use an electric drill. Now, word of caution - the first few barrels I did, back in the 70's, needed correcting to get this short 3/8" long cone straight. It is best if there is someone to keep it straight one way while you keep it straight from the other direction. Frequent checking is advised. When using power, I use an oil - anything will work. After shooting today, I'll take a picture of my .58's muzzle which was crowned this way. In that one, I use a ball that is larger than the bore and a .015" denim patch. The rifling is only .003" deep and I don't need a hammer or mallet but must a descent starter with a 'stub' knob. One smack with the palm and it's in - then, another to put it down 5" with the starter's shaft, then from there, it's an easy push down the bore to the powder. Actual loading takes only about 15 seconds or less.