For conventional small capacity, reasonably high load density handgun rounds I agree there is virtually no difference between small primers beyond potential light strike issues (with the exception of magnum rifle primers). However, in terms of light strike issues there can be differences even between brands.

For example one of my S&W revolvers has a marginal hammer spring that is 100% reliable with CCI 500 primers but has occasional light strikes with Winchester WSP primers.

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Where primer brisance starts to matter is in rifle rounds, in terms of accuracy and to some extent safety, and potentially in some of the older high capacity low load density cartridges, like the .38 Special

For example the .22 Hornet in general is an exceptionally accurate round, but many .22 Hornet shooters get even better accuracy with small standard pistol primers. It makes sense, as the case capacity is quite small, and the pressure is low enough that the lighter cup doesn’t pose any issues.

At the same time, I’ve measured average velocities and higher standard deviation in velocities using magnum small rifle primers in the .22 Hornet - on the order of 75 FPS faster with some loads and with some obvious pressure signs. In that particular instance with some powders it makes a significant difference. Enough that if I do another batch of .22 Hornet using small rifle primers, I’ll take the precaution of working up a load to my target velocity using the rifle primers.

At the other end of the spectrum, large capacity rifle cases using some of the harder to ignite powders really do need a magnum primer for consistent ignition and to avoid hang fires.

Accuracy wise, primer brisance also effects how much the powder is scattered in low density loads and too much primer with a low load density can create pressure spikes.

That increased scattering of powder in high volume cases like the .38 Special (which was originally designed as a black powder case and has way more volume than it needs for smokeless powder). Excessive primer brisance can potentially create pressure spikes with the very low load density loads you often have with some of the fast flake powders.

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The point here is don’t over generalize the results in 9mm cartridges to and other pistol rounds to the entire range of rifle rounds or think you can use a magnum small rifle primer, or a large rifle primer of any type.

(See my post on Supervel’s BYOP program below.)