Are small magnum pistol primers harder than small pistol primers?

I've been handloading 357 magnum for a couple of years now. I just recently noticed that the recipes I got from Accurate Arms for AA9 in 357 mag called for magnum small pistol primers, and I'd been using small pistol primers.

So over the weekend I fired 50 of the old rounds with small pistol primers, and had no issues. I then fired 50 rounds that had the magnum small pistol primers and had about 3 misfires. The gun was a GP100 that's never misfired before. I previously used CCI small pistol primers (#500), and on this batch started using the CCI magnum small pistol primers (#550).

It's not a huge thing, and the rounds fired on a second try, but I HATE hearing a click when expecting a bang. The only thing I can figure is that the 550's are harder than the 500's. Is it possible that the 550's are harder?

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