I have a Martini Henry in 45/70, a custom job obviously. Being an antique it has the same pressure limits as your modern trap door. I have examined both mechanisms and even built of modern materials, the trap door is not a mechanically strong design. Most lever actions have a long springy bolt, locked at the rear, they are not exactly strong either.
Anyway I have been using AA5744 which does very well in this caliber. Accurate Arms claims that it was developed for these big black powder cases, all I can say it shoots well, and does not require a filler. My data is below, I think I settled on 27.5 grains with a 405 LRN. I can keep my shots within four inches at 100 yards offhand, which is the only type of shooting I do with this rifle. I believe the velocities are in the blackpowder range, the rifle is.
If you notice, AA5744 gives excellent SD and ES’s in the 45/70. I have been using this powder in 30-06 and 303 with cast bullets, and it works better than any other powder I have tried.
26" Shilen Barrel
405 LRN 26.5gr AA5744 R-P cases WLR
T = 80 °F Group Size v. good
Ave Vel = 1294
Std Dev = 13
ES 40
Low 1274
High 1314
N = 15
405 LRN 27.0gr AA5744 R-P cases WLR
T = 70 °F
Ave Vel = 1284
Std Dev = 22
ES 80
Low 1263
High 1343
N = 12
405 LRN 27.5gr AA5744 R-P cases WLR
T = 75 °F Group Size good offhand at 100 yards
Ave Vel = 1363
Std Dev = 24
ES 97
Low 1298
High 1395
N = 20