Tom, for the muzzleloader, I am sure you will find about any twist from 60" to 80" to work fine.

The faster the twist, the more "hold" you will likely need, so deeper rifling of .010", I'd recommend. At 80" to 100", shallower rifling is likely OK & will be easier loading, in that it will allow looser (thinner patch) loading, so .006 to .008" would be fine.

The placement of a wad beneath the patched ball also helps in some loads, to prevent damage to the patch by the flame, in less than tight loads.

In such a large bore, 010" is quite shallow, however with really slow rifling, like Forsyth recommended 100" and slower, you can use even shallower rifling. VERY wide grooves and narrow lands makes loading much easier.

I am sure the 66" twist will likely work well in your Martini 12 bore and being a ctg. gun shooting oversized bullets, you can get away with shallower rifling, say .006" or so. .006" might also work in the muzzleloader, I do not know for sure.

A friend has an 11 bore Getz deep groove barrel on his English gun (Purdey Style) that my Bro built him back in '86. Seems to me the rate of twist was 60" or 66". Keith would have been happier with shallower rifling, 010" to .012" as they are easier loading and do not hold fouling like the deeper grooves do.

WW balls (his preferred moose projectile) are difficult to load with a patch that goes to the bottom of the .025" rifling - which we feel is way too deep for such a large bore.

On the plus side, that rate of twist shot well from 82gr. all the way to 200gr. At 120gr., he was able to keep a ball inside a moose, rather than have them all exit. He started at 200gr., then each year, reduced the load by 20gr. For sub freezing hunting, paper ctgs. worked well, but fouled excessively compared to my 14 bore rifle. He was only able to fire 3 with good accuracy. After than, he needed to wipe out the bore before loading and shooting more. In my .012" rifling, I am able to fire 10 without loading difficulty and their accuracy ans POI matches patched round balls.