See that's my question maybe someone with a better grasp on the whys than I can explain it; why would I need magnum primers with the 357 and just standard with the 38 specials? It's the same powder, often I'm using the same bullet (125 gn hornady xtps) and 90% of the time I'm even firing them in the same gun, a 357. Why would it be different primers?
As said, the powder dictates primer choice typically. My comment is a little outside the general discussion here but it’s still relevant given what you said and I quoted above.
If you are making .38 Special and .357 Magnum with the same powder... I would be of the opinion that in almost all cases... you are doing it wrong.
Great .38 loads demand a fast burning powder. And true magnum revolver loads are lousy imposters if you are making them with fast burning pistol powders. They will not return magnum performance and they do this will ALL the same peak pressure of a magnum load.
The only thing that magnum loads do with fast burning pistol powders that can be claimed as a benefit is that they use a smaller charge weight. So you can save literally a penny or two in powder.
I’ll say with no reservation that if you are making .357 Magnum ammo using any of Ramshot Zip, AA#2, Bullseye, Titegroup (the worst possible choice) or any of the Red Dot, Promo, 700X... you are not building .357 Magnum well.
This is not meant to be condescending in any way whatsoever, I went through this learning curve and I suspect every handloader does also. When I learned about the differences in handgun powders and how they are best used, it was a genuine
epiphany in my journey.
Last bit: if you are making .327 Federal handloads, note that the specified primer is a small rifle primer. Federal builds all of their factory .327 Federal ammo (45k psi max) with a small rifle primer.