I'm the one who spoke to the lady at CCI who stated not only that they were the same but that the CCI staff who shot 357 Magnum and the like all bought the 400's because, even with employee discounts, they had a lower price tag just because the packaging didn't have the word "magnum" on it.
However, this was many years ago (probably around the time of the shortage that started in 2009). Allan Jones, who used to develop CCI primers, among other things,
points out that primers change more often than folks realize, and it is not normally announced. So I called CCI again to see what they say now. The guy on the phone immediately said, "NO! They are not the same, and anyone interchanging them is playing with Dynamite!"
Hmmm. Unlike the lady I spoke with originally, he did not turn on a computer and look up the cups and anvils and charges used. It was a rote recitation. And "Dynamite?" That's more than a bit over-the-top hyperbole. So, while it is the current official line that they are not the same, I have less faith in the care and feeding that went into the more recent answer than I had in the original. But still, the primers could have changed since then.
So I went online and looked for tests. My preference would have been to do my own, but I did not have current production CCI primers in SRP and SPM, and unless I test current production, what's the point? This came up just before the shortage got into full swing, and I figured to be able to pick some up. So I ordered some 38 cal cast wax bullets for better weight consistency and figured to drill out some 38 Special new brass flash holes to ⅛" for firing them by primer alone to get muzzle energy comparisons. By the time they arrived, though, the primer shortage was well underway, sinking that test.
So, second best, I went looking online. I found the same YouTube video Shadow9mm linked to, which is
this one conducted by ammunition manufacturer Super Vel in a pressure test gun, comparing CCI SP, SPM, and SR primers (500, 550, and 400's). They didn't use an adequate sample size to detect a mean difference, so there may be a few fps there, but pressures and velocities all did seem to overlap, excluding an outlier each from pressure and velocity tests, and the one highest pressure reading they got was with a CCI 500 standard small pistol primer, so go figure. In any event, so much for "playing with Dynamite."
This fellow also tries these three primers in 9 mm. He also can't tell any difference by feel or by primer inspection. I just include his video because his attitude makes the most sense. He does say, as his video title does, "don't do this!" To paraphrase him, don't do this even though it makes no difference and doesn't appear to cause any problems. Still, it isn't recommended by the manufacturer, so it's not recommended here. Would I try this in a pinch? Sure. But that's just what I would do, and not a recommendation.
At one point, I found (can't find it now) a comparison done in a carbine that found some kind of difference. Was it a lot of difference, or did the long barrel make a difference? I've forgotten. I just remember the gun didn't blow up.
I will have to take a look at the Rifle article.