Last time the tooling and process of seating a primer got detailed, and now more details about The Thing Itself. Read all about it…
A primer consists of a brass (usually) cup filled with impact-detonated explosive compound, lead styphate specifically. Right. Primers explode. The compound starts as liquid, not that that matters, and while it’s still wet, a triangular metal piece called an “anvil” is positioned in the opening. When it’s hit by the firing pin, the center of the cup collapses, squeezing the explosive compound between the interior of the cup and the anvil. That ignites the compound and shoots a flame through the flash hole. That ignites the propellant.
There are two primer sizes, and then type variations. The two sizes are “small” and “large.” For example, .223 Rem. uses small, .308 Win. uses large. Rifle primers and pistol primers are not the same, even though they have diameters in common. Rifle primers have a tougher cup, and, usually, provide a hotter flash. Do not substitute pistol primers for rifle primers! Some pistol shooters using very high-pressure loads substitute rifle primers, but also often need to increase striker impact power.
Variations: There are small variations in primer dimensions, heights and diameters, and also variations exist in new-case primer pocket dimensions, among various brands, and, of course, lot-to-lot variations can and do exist within any one brand. Usually, these variations are not influential to suitability. Usually. However! On occasion, small diameter variations can affect how well different primers will feed through various make priming apparatus. This can and has become a hitch in some progressive loading machines. Cup height variations can lead to seating depth (primer height) issues.
There are also “magnum” primers. These have a hotter spark. They are engineered to deliver a stouter kick-off to larger, more dense columns of slower-burning propellant. They also work well with spherical-type propellants (less air space between the granules). There are also “match” primers. These ostensibly are more consistent quality. Not all manufacturers offer these options. If they do, unless you have a scheme or more carefully-considered reason, just go with what fits your application. There’s no need for match primers in blasting ammo. There are, no doubt (and no doubt significant) differences among varying brand primers with respect to “output.” As mentioned earlier on, there are also pretty well-known tendencies that are either more or less preferable among varying primer brands.
The primer is, in my experience, the greatest variable that can change the performance of a load combination, which is mostly to say “pressure.” Never (never ever) switch primer brands without backing off the propellant charge and proving to yourself how far to take it back up, or to even back it off more. Don’t deny this one.
I back off one full grain of propellant to try a different primer brand.
Finding the best-performing primer for any particular combination of cartridge, bullet, and propellant isn’t just always as easy as putting a “match” primer in there. I have my preference, and it’s what I try first, but, to be certain, sometimes best accuracy and consistency (related) come with another. Again, it’s a combination of propellant fill volume, burning rate, propellant type (single-base, double-base, extruded, or spherical), and column “packing” density that favors either a “hotter” or “cooler” flash.
Priming cup composition also factors mightily in my final choice, and that’s a big factor in some semi-autos. More next time.
Do be extra careful handling primers! No kidding. It’s the most explosive element in a cartridge, and it’s intended to be detonated from impact, so… Wearing safety glasses at the loading bench might seem nerdy, but it’s wise. Likewise, and this has happened way on more than once, but, fortunately, never yet to me, is a mass detonation of primers contained in a feeding device, such as a primer feeding magazine tube. Such circumstance is grave indeed. Progressive loading machines, as well as many bench-mounted appliances, use a tube magazine that contains the primers. This tube must be filled, like any magazine. Make sure you know when full is full, and don’t try to poke in one more. This is usually when “it” happens. Remember, primers are detonated via pressure. Said before, but important enough to say again now: Never (ever) attempt to more deeply seat a primer on a loaded round. And keep the priming cup (the tool part that holds the primer for seating) clear of all debris. I’ve heard tell of brass shavings, leftover tumbling media, and the like, getting between the primer and the tool cup, and forming its own little firing pin.
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The information in this article is from Glen’s newest book, Top-Grade Ammo, available HERE at Midsouth. Also check HERE for more information about this and other publications from Zediker Publishing.