Howard 31, The very first thing that should be done with any rifle that goes from the jacketed round to firing any lead bullet is a good cleaning to remove any and all copper fouling.
The next thing is that your trapdoor will do better with bullets that are in the 20-1 alloy range which have a BHN of 10.
All commerical lead bullets are way too hard for firing with BP and they also contain the wrong lube. I did see where you stated that you used Pyrodex which I don't care for but that is your choice.
If you really want to see that trapdoor perform do the following things
(1) Use real BP by KIK, Swiss, Schutezen, Goex Express
(2) Anneal all your brass case necks
(3) Slug your bore with a egg shaped fishing sinker with lots of oil
(4) Buy your bullets from a BP dealer such as Sagebrush and use the heaver offerings not the lighter ones.
(5) Use Ballistol and water in a 7-1 mix (Moosemilk) or Windex/Vingear for all your cleaning and a good oil like EEZOX
(6) Have fun like you have never seen
SASS Frontier Cartridge class, 100% BP Shooter,