I just did some testing today, loading smaoll rifle primers in the 357 Magnum, cowboy loads. They run about 800 fps, a 132 cast bullet and some Universal. I've tested this load, somewhat extensively, and experienced some unexpected results. With this load, different brands of SP and SPM primers produce varied results. The brand of brass makes a difference as well.

It is fairly common knowledge that a CCI SP Magnum primer is the same as a standard small rifle primer. Of course the SR magnum CCI has a bit thicker cup and I assume more compound, so I don't see the wherefore of using it in a pistol cartridge. Most small rifle primers use a pretty soft cup, and they go fairly flat when the pressure is up there.

I was gifted 900 small rifle primers, Dominion brand, made in Russia. I wanted to see what the numbers would be like in the 357 mag with my cowboy load. In federal brass they ran Vel/av=804, Es=30.12, Sd=8.54. In R-P brass, the velocity was lower and the stats were at least double.

With the small amount of powder and the lowish pressure ignition is precarious to say the least. It seems to have a lot to do with the shape of the cartridge floor and the punch ring left from making the primer.

Bottom line, if you are wanting to use SR primers, I think they should be similar to a small pistol magnum primer. I would think most SR primers would be "hotter" than a standard small pistol primer. I wouldn't just willy-nilly switch primers on any top load, and I would chrono to check their performance.

Primers make more of a difference on starting, or low pressure loads than when you get up in pressure. Speer manual mentions this.

Getting magnums to shoot cowboy loads is a real challenge, luckily the targets are big, and close. Typical CAS loads are app 750 fps for pistol. Some shoot ammo loaded even slower. I've heard some load a 90 gr bullet in the 357 with no powder, just a SPM primer for about 500 fps. That is not something I've done, and it requires some flash hole work to make it function.