onondaga
posted this
11 January 2017
Tooweels
If it is pressure safety you are concerned with Lee has relatively close data* in psi for a slightly heavier 350 gr cast bullet in 45-70 modern rifles. MAXIMUM 56.0 gr H4198 reaches 2300 fps @ 50,000 psi. your lighter bullet will have less pressure at the same charge and 50.5 gr is recommended START with 2091 fps. A 10 gr weight difference with cast bullets in this weight and caliber has nearly zero effect on pressure or velocity. Do not substitute IMR 4198 or your 2207, pressures are not equal but the recommended START is safe for the IMR or H 4198. Hodgdon specifically states not to use H4198 below recommended START level as ignition problems can be dangerous. There are much better igniting powders to go lower, IMR Trailboss will go much lower very safely.Following IMR general directions for Trailboss will not over-pressure your rifle.
The same Lee book also has a formula for calculating pressure change with charge reduction that is very accurate. If you run spread sheets well, you will eat up the Lee formula and spit out graduated data. A calculator is very time consuming but will work for the short span algorithm. Short span means the farther you are from base data you are the greater the error of the spreadsheet or calculator results. Safety tells me not to use the algorithm hotter than10% below MAXIMUM
* p.692, Modern Reloading Second Edition, Richard Lee
Also there is a formula for converting CUP to PSI but it is only accurate to +- 3000 units:
CIP/CUP units
(CUP x 1.20911)-2608.88 = ANSI psi within 3,000 units DO NOT USE THIS FORMULA ANYWHERE NEAR MAX.
WARNING !!!!!!!!! If you don't understand this stuff like a math teacher, don't risk your life.
Gary