I don't think you have to reduce your charge weights at all. I tried a
few loads Saturday here in IN at a balmy 40 degrees or so to test for
primer effect. The gun was my 4" S&W md 28-2 357 mag. The load
was Win brass, 7.7 grs Hdg. Longshot powder and TV 158 gr cast SWC.
Velocity is avg. of five shots at about 12 ft from my Pro Chrono digital
chronograph. Primer; WSPM = 1205 fps, ES = 42fps. Federal 200 SPMP
= 1174 fps, ES = 71 fps. Win SR used for standard or mag loads, =
1198 fps, ES = 64 fps. Rem 7 1/2 a SRM primer = 1206 fps, ES = 32
fps. Also tried another load to test for primer effect with 2400 powder.
Same gun, brass and bullet. Load was 13.0 grs 2400 with Win SP std
primer = 1144 fps, ES = 130 fps, Second load was 12.2 grs 2400 with
WSPM mag primer = 1229 fps, ES = 27 fps, and that's why despite
all the dire warnings from the chicken littles I use magnum primers
with 2400.