I read something yesterday which might apply but I found too advanced for me at this point>
Something to do with the use of a Magnum primer?? I am not familiar with .38 / .357 M but do they use the same cartridge? The Magnum primer is said to produce ~5,000 more cup so how might that fall into play with making a +P load for a .38?
If the .38 uses small pistol primers, reducing the powder charge and introducing a Magnum small pistol primer with a slower burning powder might ... well I do not know and am only throwing this out for knowledgeable comments, I am not trying it!
I actually received a box of Magnum Small Pistol primers in error at a gun show this weekend and will be offering up for sale/trade.
I am a fairly new reloader so I am asking for info not a chops busting.
You want to use magnum primers only if a load calls for them, it isn't necessarilty dictated by caliber, or even pressure, it is for certain slow powders that need a hotter spark to light them consistently, some have lots of retardant in the outer layers, and some cases are shaped in a way that they are harder to keep a consistent burn going, especially when the case is full. In most correct uses velocity isn't that much higher, but they are more consistent because the flame propagates deeper, standard primers otherwise could fire the shot, with random clumps of unburned powder flying out after the bullet. There are fewer and fewer loads requiring magnum primers though, new powders generally light easier, then burn slower, unless you have a pet load that uses magnums most reloaders rarely if ever use them. I have a load or two that call for LPM primers, although Win LP primers are hot enough to be labeled for standard or magnum loads, and they work well in every load I use, never needed SPMs though. When used as a substitute for standard primers there may be no noticeable difference, there may be pressure signs and higher pressures when there weren't any with standard primers. There also can be a difference between brands of primers too, and unfortunately some load data isn't real specific on what primer the load was tested with, part of the reason you always want to work up a load before stamping out bulk loads