I would measure the bore diameter of the 12 and use a ball that was bore diameter or slightly smaller. Jef Tanner willmake any mould size you want.

I've had excellent accuracy in both a straight rifled 13 bore with .684's (.703 groove, .680" bore) and my 12's using a gas check cup underneath the ball to keep it centred and take the rifling, or guide the ball straight down the tube of a smoothbore. Too, they work great in my rifled 12 that has a 36" twist - bit fast (for me) for RB, but they do work well. The rifled 12 also does well, but no better with the .710" ball in it's .714" bore, .724" groove diameter. both shoot a small single hole for 5 shots at 30 yards using both black powder and smokeless powder.

The advantage of RB's over Brenneke's is the RB's are harder alloy if cast of WW metal and MUCH harder if cast then either merely quenched in water straight from the mould(Brinel 30 to 34), or oven hardened - about 460F is perfect- then quench in tap water. 24 hours later they'll be around Brinel 30 to 32, which is almost 50% harder than lino, but lacks brittle nature, hense penetrates much better. Standard WW is a great penetrator by itself and really doesn't need hardening further. Out to normal 12 bore range, say 150 yards from a rifle, there is no advantage in a slug. Slugs in big bores have literally no advantage at all as they produce more arched trajectories and cannot be driven to the speed of the common round ball. The flatter the trajectory, the easier it is to hit what you want,not having to hold for various ranges but to aim true and hit perfectly. Even a good sized 12, with round ball, will give a point blank trajectory of 90 yards, where the ball is no more than 1 1/2" above or below the line of sight. For a 12 bore round ball at 7 dram velocity (about 1,500fps) and zero'd at 80 yards, it will be 1.4" high at 50 yards, and 1.5" low at 95 yards, less than 4" low at 110yards. Slugs (hollow or very short) of identical weight as RB, produce higher barrel pressure and therefore cannot produce the same velocity for the same pressure.

As written by Major Shakespear, author of "Wild Sports In India", but long as the spherical bullets go through and through large game, I do not see the use of running the risk of shaking the stock of the gun, and of extra recoil, by using the heavier balls." He was, of course referring to the minnie balls being made for the large bore guns at the time - one of which would weigh about 1-1/2oz. in 14 bore, 2oz. in 12.