With rifle loads, a primer change can have a measurable (and meaningful) affect on accuracy.
The differences in primer explosive power (brisance) is all over the map from brand to brand (
Check out these photos). As you can see, some primers are much more powerful than others, and a primer change can increase or decrease the chamber pressure in a significant manner.
Excellent link to photos EC.
Those photos are cool, and I had been thinking it was a good way to determine brisance, as manufacturers don't supply objective data. However, I would like to see the methodology, that is, of the photographing process. I can't find anything on that site that describes the set-up, as in the camera triggering method, shutter speed, etc. Without such information, and lab calibration and validation exercise, the pictures are pretty much worthless. This is so because since the ignition and combustion of the primer occurs over a period of time that can easily exceed even nominally fast shutter speeds, the camera is probably capturing a moment during the combustion and not telling the whole story. Just consider, for a moment, for those of us old enough to remember, differing flash synchronization settings on cameras, and different burn-length flash bulbs (as in FP for focal plane shutters, much long burn than M, F, for example). In addition, I would guess that "free air" performance of primers differs from performance contained in a cartridge case, either full of powder (think rifle), or with mostly air (think light pistol loads using Bullseye); though, it's a good place to start. It's a shame manufacturers don't provide objective performance data.