[Unclenick]

FizzyP,

Trail Boss is just different. It's designed to run at low pressures like a modest dose of pistol powder, but its ignition behavior doesn't quite track regular powders. Part of this is because of all the empty space the donut holes leave in the case. Going to a magnum primer (they make more space-filling gas than a standard primer does) can often cut its velocity variation for that reason. Also, a really strong crimp is a big help. I've been favoring the Redding Profile Crimp Die for this. Its crimp starts as a taper crimp but terminates in a roll crimp. This keeps the sharp roll from deflecting brass away from the sides of the bullet to have better friction.

Note that H4198 loads for the trapdoor are lower in velocity than the IMR4198 loads, but produce more velocity variation. This is due to empty space again, and, again, a magnum primer can help with that. So can other cast bullet shooting techniques like putting a tuft of Dacron pillow ticking over the powder before seating the bullet, to keep the powder against the flash hole.