The Greenhill formula gives the slowest twist for stability, not the optimum. Too, it doesn't work well for round ball guns. A bunch of the guys use different 'constants' insted of 150 number. Try plugging in 120 and see what happens - or maybe it was 170? Anyway, it doesn't matter. We know 70" to 80" works better than 100" or slower. The slower twists will give descent hunting accuracy, but require very large powder charges to get enough velocity to make the twists work and are not realy flexible with lower charges.
: Trust me on this, 80" is a great comprimize twist and will work just fine. I would stay away from faster twists. I know a 14 bore ball(.680" to .690") will shoot well with 66" of twist, but it required a very tight combination to maintain patch integrity and prevent excessive fouling and mainting accuracy. This was in a muzzleloading rifle and it-too demanded fairly heavy charges to shoot accurately. Best accuracy was derived from 4 1/2 drams to 7 drams - even in it's fairly fast twist for the bore size.
: Since you're using a breech loader, fouling is more easily removed when and if it happens. Next time I get a bore barrel, from 16 bore to 12 bore, it will be in 75" to 80" twist.
: Forsyth's recommendations for a 1/4 turn in the length of the barrels (26" tubes) as being the best for hunting, takes into consideration they used very shallow rifling a a weak combination of ball and patch. More twist would cause stripping and excessive fouling, but the roughly 102" to 104" twist would still give minute of elephant, tiger and bear accuracy at the very close ranges they shot in the jungle with heavy charges. We know today, a slightly faster twist gives even better accuracy, and that about .006" is plenty deep enough to work well. My 14 bore rifle made normal 100 meter 5 shot groups of 1-1/2" or smaller with many of those in the 1.2" to 1.4" range which shows 66" isn't too fast, just too fast and requiring too deep rifling for loading a 12 gauge case with plastic wads. Fouling is a bug-bear when plastic is used. It must be well protected from the black powder flame if it is used. For this reason, my friends use cloth patches on the balls loaded in their duck guns (moose loads). So- either protected plastic or cloth patching as used by muzzleloaders will work. If using cloth, make it a very tight fit inside the shotshell. You want the patch stripping off the ball as it leaves the shell.
: You can soak a fibre wads in a mix of 40% Vaseline, 60% beeswax for a good lube (then sweeze them out slightly), or just use a comercial lube like "SPG" or Lyman's "Black Powder Gold". Even though Vaseline is a petrolium derivitive, it works wonderfully with BP fouling. Must be the heat and wax that changes the normal petrolium and BPfouling's non-mixing properties. I've come to prefer that lube for BP ctg. shooting.
: I hope this further explains BP loading in shotshells with ball.
: Remember, with smokeless powder charges, plastic can be on the powder as in all modern shotshells. With black powder charges, you need a barrier between the plastic and the powder.
: Something you might also try, if you get the inclination, Longknife, is the Lyman Sabot slug. In WW alloy, this slug weighs 508gr. or thereabouts and fits perfectly in a Red WW Field wad. Corresponding powder charges can be found in Lyman's shotshell loading books. Cast in pure lead, they are supposed to weigh 525gr. When seated inside the top of a Red Wad, fingers and base cut off, this slug can be loaded over card wads in a black powder loading. I'd use it in pure lead for deer or medium game, but WW alloy for large game like ELk, moose or big bear. Friend Carl in Alberta says this slug out-penetrates Brenneke's by a good margin.
: Lyman also has a 345 gr. sabot slug that fits in a 20 bore wad for use in 20 bore guns. Their shotshell handloading books also supply data for this one.