What you can do is e-mail Federal and ask them what is the difference between their standard & magnum pistol primers - I think their answer will be cup thickness. A thicker primer cup with high pressure loads will greatly assist in preventing primer piercing. I don't believe there is any other difference between the Federal magnum & standard large pistol primers.
I also believe the WLP primer has the thicker cup, so it can also be used in high pressure rounds calling for a large magnum pistol primer.
The only U.S.A. manufacturer that I'm aware of that makes a "true" magnum pistol primer is CCI - their 350 is one hot puppy and your peak pressure will be higher as well.
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