Tom- you should use the .715" ball in a 12 bore. The larger the ball is, that you can use, the better the accuracy will be. For instance, in my .69, I can get away with using a .675" ball which is .015" smaller than the bore if I use a VERY heavy patch. For that, it takes a .030" denim patch - 12 oz. the heaviest denim for clothing they make, I assume for jackets and work coveralls. The muzzle of my .69, is the lowest picture, showing some rod wear at 100 o'clock. This does not hurt the accuracy.

Patches fray around the outside, due to the muzzle blast. What is important, is where the rifling presses against the material. Brown streaks radiating out beside the land marks shows gas blow by. The base of the patch should still have some lube on it, not be very dry and scorched.

Some guys put a patch down on the powder to separate the powder charge from their 'wet' patch. some guns allow this wihtout a loss in accuracy, some don't. The larger bores are usually much less fussy than a small bore. Nothing new there.

You can try using 2 patches, but better accuracy will come from one and a larger ball.

Now - you may get OK accuracy, with a small small and that's OK.

If you are casting balls, you can use harder alloys with the smaller ball & thicker patches that cannot be loaded with a larger hard ball.

When a ML double crosses, it usually means it needs more powder. Go up in no more than 10gr. incriments. The tubes should uncross at some level of charge, then open to parallel - possibly - whether it's 110gr., .120gr., 130gr. or whatever.

My own Kodiak .58 did this for me, but shot 'true' with smaller charges than I would have liked.

Here is a short video - many of you have probalby seen it - sorry for the repeat. In it, I am using a .562" ball and a .0215" pre-cut round ticking patch. I have since changed to a .573" X .574" pure lead ball (old Lyman Ideal .570" mould) and use a 10oz denim patch, at .022". The .0215" ticking shot as well, but I ran out of it. The other shoots and loads identically.
With the .562" ball I could get double barreled groups of 1 1/2" to 2" group at 50 yards off a rest- ceretainly good enough for hunting. The larger balls make a nice 1" group, quite consistantly. There is no down-side to the larger balls that I can see.

Note, I load the right barrel with one stroke of the rifle's 3/8" hickory rod, then say to choke up on it and I load the second barrel using several strokes. After the ball is down just off the powder, I place the starter's knob (w/3/8" hole) on top of the rod and give it a final smack. That is to put the ball onto the powder with virtually exactly the same pressure each time. I do this the same every time I load the gun. Consistancy is 50% of a ML rifle's accuracy- it is said. Works for me.

edited to include the video