I already convered some differences between a 12 bore loaded with a solid round ball. There is a bit of difference between pure lead and hardened lead. Balls hardened with antimony, such as using Wheel Weight metal, or a 50/50 mix of pure and WW metal, gives the needed strength to punch thourhg bones, large and small without expanding. I've used hardened 14 bore balls for just such a purpose on shoudler bones of 1,000 pound moose, without damage to the balls and excellent penetration. My good friend, with a .75 cal rifle, had trouble keeping a .735" WW ball inside a moose, until he reduced his load to 4 drams.
: With it's almost same sized balls, the 12 bore has similar results, ie: spectacular.
: The modern 12 bore slug loads, those with 1 and 1-1/4 ounce hollow based slugs, shoot fairly well, but not generally as well as properly loaded round balls. The "pumpkin balls" of yesteryear, gave the same sub-standard accuracy as the common musket did of the 18th century and for identical reasons - very much undersized balls. My own load developement with round balls in the 12 bore showed excellent accuracy, good enough for deer shooting to a full 100 ayrds, and that from a side by side, lefts and rights, hitting the same poi.
: As most of the round ball smoothbores in 8 bore were of short barrels, I'd not push the matter with outsized shot barrels. Therefore, I'd reduce the lengths to a more comfortable 22" or perhps 24". Note that Greener's suggested weights were around 11-1/2 to 12 pounds for an 8 bore shooting round balls.
: Properly measuring the bores will show the size balls needed. I'd keep them as close to bore size as possible, yet sill able to load in brass shotshells for that bore size. Experimentation with cushion, lubricating and card wads, along with perhaps cloth patching the balls will eventually reward the shooter with the required accuracy.
: J Tanner of England can make the requisit mould and they're of excellent quality, as the two I have are.
: A google search will give you his online address along with pictures and instructions for ordering one of his brass or bronze moulds.