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I have tested both Federal and CCI regular and magnum primers in 30-06 and smaller cartridges, and have never been able to detect any difference in pressure or muzzle velocity.
At some point, a cartridge is big enough that I'm sure the primer will make a difference. But I haven't found that point yet.
I have a suspicion that the belief that some primers cause more pressure than others is nothing more than failure to properly account for the natural variation of the process, sloppiness in controlling the other variables, or failure to properly randomize the test.
If you shoot 20-30 shots, at about 1 minute intervals, you will see a pretty linear increase in MV, just from the increase in barrel temperature. If the first half your shots are Apex primers, and the last half are Blivet primers, you could easily conclude that Blivets are hotter, when, in fact, it might have been just barrel temperature.
The energy of a primer is too small to cause much difference in pressure. What it might affect is how rapidly the powder catches fire. As nearly as I can tell, the ideal is to shoot a column of fire up the centerline of the cartridge, so the powder burns from the center out, rather than the rear forward.
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