Priming compound, the material with which the primer cup is filled, is pressure-sensitive and ignitable, in other words, you hit it and it burns. Priming compound is a complex blend of chemicals designed to accomplish different things, but for purposes of this discussion we’re interested only in what makes the compound pressure-sensitive and ignitable, which is called the initiator. The history of primers over the centuries really is mostly a story of changes to the initiator.
The earliest primers, going back to muzzleloading days, used mercury fulminate as an initiator. This actually worked really well in such guns, because it was stable and reliable. The problem came with the development of brass cartridge casings: mercury residue from the fired primer would attack the brass, in short order making it extremely brittle thus unsuitable for reloading.
Eventually we got around that by developing primers using potassium chlorate instead of mercury fulminate as the initiator. Unfortunately we exchanged one problem for another. While potassium chlorate residue does not attack brass, it attacks steel because the residue contains chemicals similar to common table salt. Thus the bore (hole through the center of the barrel), and parts near it, as well, would begin to corrode very shortly after being fired with potassium chlorate primers, unless they were cleaned immediately with a solution of hot, soapy water. For this reason, potassium chlorate primers are called corrosive.
In the late 1920s came primers using lead styphnate as the initiator. This is what’s still used in the vast majority of primers to this day, because it solved all the problems. Lead styphnate residue does not weaken brass, nor does it corrode steel. Therefore lead styphnate primers are called non-corrosive. The idea that a shooter must immediately clean their gun, especially the bore, immediately after every shooting session in order to avoid damage is a holdover from pre-lead styphnate primer days. I’m not saying you should never clean your gun, just saying the idea you must clean it immediately after firing or it will begin to disintegrate is obsolete information. Lead styphnate primers got us past that.
Though commercially sold ammunition went to the lead styphnate system en masse shortly after its development, the American military stayed with potassium chlorate til the 1950s. Such ammunition can still be found on the surplus market even today. Thus, when buying cheap military surplus ammunition, it’s very important to ascertain whether it’s loaded with corrosive or non-corrosive primers.
In the early 1990s, due to concerns over airborne lead when firing lead styphnate primers on poorly ventilated indoor ranges, primers were developed that are lead-free. For multiple reasons beyond the scope of this article, lead-free primers are only available in factory-loaded ammunition; they’ve never been sold as reloading components. Unless and until lead-free priming compounds change and improve dramatically, they never will be. Thus for handloaders, lead-free primers are a non-issue, but I mention them here simply in the interest of completeness.”
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