I am leaning of being persuaded of the same thing, at least for CCI. I originally got the information about the sameness of CCI small pistol magnum and standard small rifle primers (CCI 550 and 400) over the phone with CCI before Vista Outdoors bought them. The lady who was answering the phones actually took the time to look at the primer drawing and the cup and anvil dimensions and the amount of which priming mix was in them and said it showed they were identical and added that, indeed, CCI employees would buy the lower-priced 400s for both purposes themselves. But when I called them about this more recently, the technician said, "no, mixing them up is playing with dynamite!" But he didn't look anything up.
I've shot enough different primers in things to know "dynamite" was in exaggeration. Indeed, the biggest performance difference I've seen was in a test Charles Petty did in 2006 of small rifle primers going from standard through magnum with many brands. This produced about a 5% span in velocity which would correspond to about a 10% change in peak pressure with a 55-grain V-max and 24 grains of Reloader 10X. SAAMI standards allow individual rounds in a ten-round average to get as much as 18% over the MAP limit, and CIP standards allow 15%, so that 10% number is within expected safe limits and is still below proof pressure (though a steady diet of such loads would accelerate throat wear).
While I thought the CCI tech was exaggerating, nonetheless, I didn't want to ignore a company warning without some proof. So I bought a bag of 38 caliber commercially molded wax bullets with the idea of making velocity comparisons for them driven by the two primers as an energy measure. I also have an analytical balance and can get the before and after weights of the sample primers to the nearest tenth of a milligram. There is just one problem: the 550 and 400 primers I have are over 20 years old. I need current production to test if it is true today, but I've had no luck finding any to buy. I still keep an eye out for them, but you all know what the primer market has been like, and I'm not going to get a second mortgage to fund the test, so I am waiting to catch some at a reasonable price.
Meanwhile, board member Hounddawg found
a fairly recent (Jan 1, 2021) YouTube test of 9mm ammo loaded with CCI 550 and 400 primers from the new style boxes. It is done by a shop with an Oehler 43 on an in-house pressure gun that performed the comparison and found no significant difference in either pressure or velocity produced by two primers. The function in a pistol was the same for both. However, the guy doing the test runs only the CCI products and runs a smaller sample than I would like to see, and then makes a blanket statement about all small pistol magnum and small rifle standard primers being the same. That's going too far without testing them all.
So, I would still prefer to do my own test to get the cups and anvils' weights and deduce the expended priming mix weight to be more thorough. But at least for this particular brand, assuming the video test result holds up with a larger sample, it looks like the CCI difference in these two primes is in name only. My assumption would be that since there is now a central phone system for all the Vista brands, there is a rule handed down from corporate headquarters to cover how all technical employees are supposed to answer the primer substitution question in all their companies, regardless of primer brand. This would make some sense as developments are changing primers down the road. Federal's new Catalyst primer, for example, and other changes in primers due to environmental laws may be in the offing. Having a consistent policy rather than an array of them avoids the potential to have one group get it wrong.
Again, this is all Internet information, so don't bank your life on it being true. Run your own tests.
This video shows why. When I finally find 550s and 400s of recent manufacture, I'll run my test and report the results.
