The point of trailboss as I understand it is to have a high initial pressure to obturate the projectile and seal the bore (ideal for cast bullets). That initial pressure spike last only a very short period of time and the powder burns fast. It's the duration of that pressure that pushes a bullet to higher velocities , imr4198 is slower and maintains that pressure longer, that results in higher velocity. It's not pressure that results in recoil, it's the velocity the projectile exits the muzzle with and the weight of the bullet. The amount of pressure remaining in the barrel when the bullet exits is what produces most of the sound from shooting - like popping a champagne cork. Faster burning powders don't have a lot of pressure remaining by the time a bullet exits, slow burning powders may still be producing expanding gas.
As far as the 45-70 derringer , I'd use the fastest powder that's appropriate , otherwise fireballs and unburned powder will likely result, erratic performance too.
I may have this incorrect but it's my understanding of it , I've got an open mind and if anyone disagrees please educate me.
