MG, you are correct. I meant small pistol magnum and small rifle. My fingers jumped ahead. I've fixed the original post and quotes. This is why you have to be wary of online information. We all typo or pass brain gas at one time or another.
I did, as 74A95 says, speak to them again last year and got a totally different story from the first time I spoke to them. However, the first time I spoke to them, the lady on the phone took the time and trouble to use her computer to double-check that the cup, anvil, priming mix, and priming mix quantity all matched between the SR and SPM. In the newer conversation, no computer checking was done. The fellow on the phone just immediately proclaimed you should never interchange them and people doing that didn't know what they were doing and were "playing with dynamite". He was quite emphatic about it.
So I decided to test some of the newer production of these products for myself by firing wax bullets with them to get velocities and also doing before and after weight measurements on my analytical balance to see how they compared. Unfortunately, the primer shortage closed its doors about two days before I could get my hands on new production samples, so I had to put that experiment on the back-burner. But I wasn't the only person thinking about testing this. In January,
this YouTube test by a loading business of a few rounds of 9 mm fired with both primers as well as standard pistol primer and taking pressure and velocity measurements was done. In his load, he could not identify any difference in performance between the three. Certainly, no "dynamite" was involved.
But
in this other test with 357 Magnum and a slow powder, some difference was apparent, indicating the SR primer got the load about 10-11% higher in pressure (based on QuickLOAD modeling).
So now the question becomes, how much of the observed difference in the second case is due to the powder involved, the primer involved, lot variation in the primers, and so on? Is the larger primer in the 9mm just getting the bullet unseated and further forward in such a short time that the expansion difference accounts for leveling the pressure? Is it that you only see the difference in hard-to-ignite powders? Or is it just primer lots involved? I don't know. The second test, assuming no big lot variation, suggests these two primers are no longer exactly the same, but the two tests taken together suggest that whether or not you can discern the difference depends on the application.
As always, start low and work up.