Primers are often made by hand mixing the wet, putty like compound on a flat surface with a flat bladed trowel. The uniformity of the mixture is critical to its shot-to-shot ignition uniformity. Almost a "black magic" process, it is not easy to do it right.
After being smeared in the holes in a metal plate, then dried, the priming pellets are put in cups then the anvil gets pressed into them. They're tested by a known force firing pin system. Those with more uniform flame are sometimes dubbed "match" and are supposed to be better for accuracy.
A friend of mine some years ago put some BB's in primed .17 Rem. cases then shot them through a BB gun barrel to measure their muzzle velocity. The speed of the BB's was directly related to the energy put out by the primer. Primers shooting BB's having the lowest velocity shot the most accurate in regular rifles. These did not necessarily have the lowest velocity spread which indicated the most uniformity of their priming compound.
I've most often used RWS 5341 primers; cost the most but also the mildest large rifle primers made. In contrast, the RWS magnum large rifle primers may well be the hottest one around but they didn't shoot as good in my 30 caliber magnums as the 5341's did.
Most primers deteriorate over time and become less uniform in their detonation characteristics. PMC primers were very uniform and on the milder side but had a near infinite shelf life. A favorite for years with long range shooters, I don't know if they're still popular.
The first batch of ammo loaded with Sierra's then new 155-gr. 30 caliber Palma bullet used powder from the same lot for the few hundred thousand rounds made. But several lots of Federal 210M match primers were used. Accuracy from a couple of those lots of primers was not nearly as good as the others. Evidence that there was considerable difference between uniformity in their "best" primers.
