BP loads usually pattern 1 choke tighter than the choke designation. This goes for BP loads in ctg. guns as well. Did a bunch of testing, ie: patterning - 30" circle, 40 yards - counting pellets in, pellets in the patterns for average percentages- it's a lot of work, but showed in spades, the inconsistancies of some loads & terrific shot to shot copnsistancy of others. It's pretty much all in the wad column.
Seems modified is about the tightest that's easily loaded from the muzzle. In 12 bore, modified is about .020 constriction. The shape of the choke, length of forcing cones, etc, also makes a big difference in patterning. BP is less critcal than smokeless in modern guns.
I turn all the wads sideways at the muzzle when loading - only way I can get them in. A large diameter tulip shaped knob on the end of the loading rod helps turn them back and fold out the 2 bent edges. A lubed fibre wad helps considerably with fouling. The chokes allow a 'normal' wad column, 1/8" hard card over powder, 1/2" lubed felt, shot, then 1/16" over ("B") shot wad.