Magnum primers are only needed for powders which are hard to ignite like W296/H110, HS-7, HS-6 and the like. They work better when lit by a magnum primer.

Unfortunately load manuals these days use SPP for .38 Special and other "standard" cartridges while they use SPM primers for anything that has a "magnum" name. I wrote Hodgdon and asked them why and they said they use and list a magnum primer for cartridges like the .357 Magnum because even if the powder doesn't need one because they are concerned some reloaders will not read the primer used and assume a magnum primer is required. That may raise the pressures making the max powder charge no longer within the max pressure limits.

If you have older manuals like the Speer 8 or 9 look at the data for the .38 Special using HS-6/W540 and you will see a magnum primer was used in the load development.

Unique and 2400 are very old powders developed before magnum primer were made. They seem to perform better with a standard primer over a magnum primer. The SD and ES numbers are lower with standard primers so that's what I would use, even with 2400 in the .357 Magnum.

I hope this helps a bit...

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