My Waksupi-built Isaac Haines .54cal. flint lock rifle is a joy to shoot and ultra reliable, but sadly I haven't yet mastered it, i.e., the flintlock. I know it's more an issue of practice than component and load choice, but I've had a devil of a time trying to figure out which round ball diameter and patch thickness the gun really liked. After too many years of only so-so results, I think I've determined what shoots best. To wit, on Tuesday, I went to the range and bench rested two RB & patch combinations: a Lee Precision .535" RB + .014" pillow ticking patch v. a RCBS .530" - .531" RB + .018" pillow ticking patch, both cast (and weighed) by me and of "pure" Pb. The charge was 60gr. Diamondback FFg. All patches were lubed with 1 Ballistol : 6 H2O and were just shy of dripping wet. Below are the results (pdf images) of each combination @ 25 yd. bench rest. I should add that the .530" RB + .018" patch was a pleasure to start and seat whereas the larger ball - thinner patch was just the opposite. In short, I think I've finally found what the IH likes.
Btw, I had similar problems with my .50cal. Lyman Great Plains cap lock, but managed to overcome them enough to do quite well in the NMLRA postal matches several years ago.