If patched tightly enough, they should shoot fairly well in a rifled 12 if not pushed too hard. Most modern rifled 12's have twists appropriate for shooting conicals, not round balls. A twist of 66" or slower will work well with round balls - in fact, would be excellent. With RB's the slower the twist, the faster you can drive them. Of course, recoil will increase.

I would use a thin denim like 6oz. rather then a weak cloth like flannel, but- depending on ball size and groove depth. The denim is much stronger. Here, denim fabric is available by the meter in 'weights'. They measure something like this, using a michrometer - 6oz= .013" ; 8oz= .017" to .018"; 10 oz.= .020" to .0215" and 12 oz= .0235" to .0240". The variations are due to different makes and slightly different weaves. For lubrication, if using black powder, use a thinned black powder bullet lube like SPG or Lyman's Back Powder Gold thinned with 100% neetsfoot oil or olive oil - or merely beeswax/olive oil about 50/50 - whatever viscosity you want, depending on temps. The lubed patch in the top of the 'shell' will also seal quite well against moisture intrusion.

I use the 10oz denim for both my muzzleloading rifles and also used it for the patched ball loads for the 16 bore's, 13 bore straight rifled barrel.(yeah- sounds strange) I've 25 rounds ready for testing, but haven't been to the range yet - it's snowed in and I hate packing a bunch of stuff when on misery slippers - (unlike 9.3x57).