I played around with fffg APP and black powder last night:
T/C New Englander, CCI #11, 295 Gr Powerbelt HP, Swab every other shot, pick nipple each shot, shot APP first.
A 10% reduction from FFG (100 FFG to 90FFFG)with APP led to similar groups about 1" in low. One misfire.
A 10% reduction (roughly, 90 FFG to 80 FFFG) with black powder, expanded my groups and moved them 2 inches higher. I did not use a chronograph. No ignition problems. Even with a 10% reduction in volume
it appears I had higher velocity. No Chrono to prove it. The gun is rocking though, and I may drop to 70 grains.
Casual observations are that APP FFG and FFFG shoot similar pressures and velocities, but without a chrono it is not science. By comparison the BP produces greater pressures with finer powder by a larger margin.
I guess I need to deal with the mess of BP or Pyrodex if I want to avoid ignition problems in a sidelock. I mostly use it as an extra when the kids or company want to shoot or hunt. I tried musket caps, but the hammer will not fit over the nipple. I even ground down the nipple, but did not have success. I can't find a Musket hammer for a T/C New Englander.
I guess I have just got lazy and spoiled with the quick clean up of APP.