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The Speer reloading manuals I have are the number 10 dated 1987 and number 13 dated 1999. The number 13 uses the psi, transducer method. My original Speer number 10 is an early version probably with a date of 1979/1980 but can’t find it presently.
The later Speer manual no longer lists the fast powders such as Bullseye and 700-X for a JHP 125 grain bullet and the older manual does not have the 38 +P at all. My decades old load of 6.0 grains of 700-X for a 125 grain JHP is lower than the window of 7.8 – 8.3 grains listed in the number 10 manual. According to the number 13 manual my 6.0 grains is 1.0 grain above 38 +P. In recent times I got data from the Hodgdon site for the 125 grain HDY XTP using 700-X. Their max load is 6.4 grains with a psi of 30.4K, that’s within the limit of 35K psi for 357 Magnum. So even by the transducer/psi method I’m OK, I think.
Best to be conservative with fast powders like Bullseye, W231 and 700-X for 357 Magnum for the 124 grain bullet, IMO. This is a maximum bullet weight for a jacketed bullet for this cartridge from what I can tell.
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