First, the Hodgdon data:
Hodgdon
Powder Trail Boss
Bullet Diameter .357"
C.O.L. 1.610"
Starting Load
Grains 3.2
Velocity (ft/s) 754
Pressure 16,500 CUP
Maximum Load
Grains 4.2
Velocity (ft/s) 865
Pressure 20,400 CUP


Second, a "loader mentor comment":
From your comments, I infer you have noticed the max pressure on the Hodgdon data is 20,400CUP, and you want to extrapolate data MORE than is listed and GUESS what the pressure will be. Seems reasonable, right?
My advice is NOT to EXTRAPOLATE nor GUESS on loading data, due to non-linearity of pressure curves.
Personally, if I wanted to go faster I would pick another powder from the loading tables that has a tested load for the speed I want.

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Science plus Art