By Duane Thomas
When we say “pierced primer” that means, when the round fires, instead of the primer remaining intact, a hole forms where the firing pin’s hit it. Let’s walk through why that might happen, why it’s a bad thing, and how to prevent it.
WHAT SHOULD HAPPEN
1. The firing pin hits the primer.
2. This impact dents the cup inward.
3. This dent presses the priming compound between the cup and the anvil – a metal insert with a flat top and trefoil legs.
4. This impact energy causes the priming compound to ignite.
5. Hot gases and flame burst through the gaps between the anvil’s legs and through the flash hole into the interior of the cartridge case.
6. The flame ignites the powder in the case, building chamber pressure that forces the bullet down the barrel at hundreds or even thousands of feet per second.
7. The primer cup, now slightly indented, stays intact and, surrounded and sealed by the primer pocket, contains the pressure.
WHAT HAPPENS WHEN THINGS GO WRONG
1. The firing pin hits the primer and the impact overpowers the cup. The firing pin strikes the cup, just like normal. But if the cup is too thin or soft, it doesn’t just dent—it deforms excessively.
2. The priming compound ignites, again just like normal, but the structural integrity of the weakened primer cup is already compromised.
3. This dent presses the priming compound between the cup and the anvil.
4. This impact energy causes the priming compound to ignite.
5. Hot gases and flame burst through the gaps between the anvil’s legs and through the flash hole into the interior of the cartridge case.
6. The flame ignites the powder in the case, building chamber pressure. The bullet still exits the bore, but back-pressure pushes rearward against the weakened, thinned-out spot in the primer cup. By now, this area is severely stretched and may already be cracked.
7. The cup ruptures at the point of impact. The firing pin’s indentation tears open into a full hole, sometimes nicely circular, sometimes irregular or jagged, as the metal gives way under the combined stress.
8. High-pressure gas escapes from the hole and leaks back through the firing pin hole, past the firing pin, and into the gun’s action.
9. High-pressure gas and small pieces of metal hit the shooter in the face. This would be a REALLY good time to be wearing shooting glasses.
Now, it’s not always this dramatic. Frequently the tip of the firing pin does a sufficient job sealing off the gas pressure, and we don’t even know we had a pierced primer until we’re picking up our brass afterward. Still something we would like to avoid.
Why do pierced primers occur? A standard-pressure pistol primer’s brass cup has a tensile strength (typically 60,000–80,000 psi) far above the chamber pressure it’s meant to contain. However, primers that maximize sensitivity typically have a cup thickness around 0.017”. This means they can actually do their job of deforming enough to mash priming compound between the cup and the anvil, even in guns with actions that have been tuned to reduce trigger pull weight, which also generally lightens hammer falls, thus firing pin impact energy. When a thin, impact-sensitive primer cup is stretched and compressed beyond its yield point by the firing pin and back-pressure, it tears, resulting in a hole where a normal firing pin dent would be.
It’s important to realize, the piercing isn’t the cup collapsing, it’s a localized failure right at the point where the firing pin concentrates its force. In handloading, pierced primers are generally a sign the handloader mismatched the primer to the cartridge. Not to give Remington a black eye here, but probably the most-common handloading-into-pierced-primer combo is loading 9mm with their small pistol primer, type name 1½.
High-pressure rounds – in pistols think something like standard-pressure 9mm at 35,000 psi max chamber pressure, or even higher with +P and +P+ loads – are more prone to pierced primers. Remington 1½s famously pierce when used in such ammo. The cups in Remington 1½s (0.017”) are thinner even than the famously sensitive Federal #100s (0.019”) and certainly more so than Winchester standard-pressure small pistol primers, sold under the imaginative name Small Pistol Primers, and CCI #500s (both 0.020”), therefore the Remington 1½’s safety margin is slimmer.
There are other factors that can contribute to pierced primers. An overly sharp firing pin makes this more likely to happen. If a load exceeds SAAMI specs (for instance an overly zealous 9mm handload), even thicker cups like Winchesters or CCIs might pierce. High pressure can cause the primer metal to flow into the firing pin channel, thinning the cup further and making piercing more likely; as the action unlocks the material trapped in the tight firing pin hole sheers off.
Magnum primers feature thicker cups than standard pistol primers and suggest themselves for high-pressure loads. Good news, Remington does make a thicker-cup small pistol primer called the 5½. Though not marketed as a Magnum pistol primer, that’s what it is. Bottom line, stay away from Remington 1½ small pistol primers in high-pressure loads like 9mm or .357 Magnum. If you must use Remington primers in such ammo, go 5½ instead.
The truth is standard primers generally work fine up to the stated SAAMI maximum chamber pressure for the cartridge they’re being loaded into. The Remington 1½s not standing up to standard-pressure 9mm is an anomaly, not the norm. Generally, we can avoid pierced primers altogether if we mate the primer to the load – avoid primers that prioritize sensitivity over strength – I’m looking at you, Remington 1½s – in high-pressure cartridges, keep our handloads with book limits, and we’re down the road.
