Hi Ed - great minds think alike. Harry (ALR) and I have been discussing just this sort of project.
: While a 95" twist would work, it's slower than necessary due to the limitations of powder capacity in a 12 bore 2 3/4" shell & subsequent recoil of heavier loads. I suspect a 3" would hold an easy 8 or 9 drams - someone else can shoot it - not me. By this, I mean a 70" to 80" would work just fine. The speed attained with be around 1,550fps to 1,600fps with 7 drams(191gr.2F) GOEX, 1,400fps to 1,450fps with 5 1/2 drams (150gr.2F) the moderate African load as listed by Greener. You can get those same speeds by using several varieties of smokeless, with greatly reduced recoil.
: I have some 4227 loads to test before publishing them here. These were suggested by Huble - the author here, of "12 Bore From Hell". right now he's working on an 8 Bore From Hell chambered on a P14 or P17 Enfield.
: Rifling depth of .010" is more than necessary and could easily be only .005" to .006" with 70" to 80" of twist. This is due to the low velocity and relatively slow twist. I would not go any faster than 70" & might finalize with 80" due to the shallow rifling and plastic wad taking the lands. Even shallower rifling would work if the twist was slower yet, of course. How to get those slower twists of 100" to 120" is another matter, but I'm sure .002" to .003" would suffice for rifling depth.
: For 70" to 80", a ML barrel could easily be used as a guide with a lead lapp cast on the rod for a guidance down the tube. A 36" to 42" barrel would be perfect as the rifling head on the other end of the guide rod could be removed from the the tubes, without removing it from the guide barrel making cleaning and oiling the cutter head easier. Extra long lengths of drill rod can be purchased from outfits who supply tool and die equipment.
: A .719" bore is small for a 12. The one I played with some years ago, was .725", also small. Nominal 12 bore is .729" with many guns today being .735" to .740".There is nothing like tight consistancy, eh? A groove diameter ball would weigh just over 500gr. I have a mould that casts .722" in WW - probably .719" in pure lead. Due to the cost of pure lead, I'd use WW alloys.
: As you suggested, there are several ways of going about loading balls. One of the worse things to have to do, is to remove lead from a barrel - shotgun or rifle. We still need to spin the ball in the rifling, though. This can be accomplished with a patch as you're used to, a cupped wad to hold the ball int he middle as the wad takes the rifling which is the system I used in my previous double smoothbore, or a groove diameter ball. The fitted, or groove diameter ball will probably lead the rifling in only a few shots due to having no grease grooves. Rolling it in a lube might help reduce that, along with lubed wads behind, but I'd rather do one of the first two. Using a trap range pick-up spend wad with a cupped base (gas-check base) is the easiest. I just snip off the wad's fingers and column with side cutters and save the gas check portion. When seated in the case with the cup facing upwards, the cup holds an undersized ball perfectly. The force of accelleration up the bore will hold the ball inside the cup, centred in the bore, meanwhile the cup takes the rifling and spins the ball delivering the requisite accuracy. With BP loads, the plastic must be isolated from the BP charge or the flame will melt plastic onto the bore. With smokeless, this is less important. A couple circle-fly card wads on the powder will do the trick. Other wads or more of them are used to get proper wad column height for crimping. You can get a roll-crimper from Lyman, but I preferred a folded star crimp. It's really exciting to watch a pheasant get hit with a ball by mistake. ie: ball loads should be marked.
: With a full case of BP + card wads, the ball did not allow a full star crimp, but the case folded forward over the front end of the ball in a coned sort of point. These never opened in the slightest and worked just fine.
: I suggest you try a .690" Lee mould (cheap) or a .690 Lyman mould (better) and go with the wads for taking the rifling or a patch, which my buddies use in their duck guns.