The Least Reliable “Pressure Indicator”

From Gun Gack III: Once More into the Breech
An excerpt from Chapter 30:
“The Least Reliable “Pressure Indicator”

In his classic 1947 book Hatcher’s Notebook, recently retired Major General Julian S. Hatcher wrote about his almost 40 years in the U.S. Army involving various aspects of firearms from handguns to artillery. Hatcher had an enormous amount of experience, including developing a solution to the brittle receivers of early 1903 Springfields, a simple hole drilled in the receiver to relieve gas pressure from blown cases. After retirement from the Army, he spent a number of years on the staff of the NRA’s magazine American Rifleman, where he provided technical information on new developments in sporting arms and ammunition as well.

Hatcher’s Notebook has been reprinted numerous times, because it contains a wealth of information still very pertinent today, including this comment:

“In the years just before World War II, it became the fashion for gunsmiths all over the country to originate ‘wildcat’ cartridges by ‘improving’ some factory cartridge, and then giving it a fancy designation with the originator’s own name added…. Usually in the process the shoulder was made considerably sharper than it was in the original….

“Making the shoulder sharper does, of course, add powder space, and thus make possible higher velocities, accompanied of course by the inevitable higher pressure…. Very carefully conducted experiments, using chronographs and pressures gauges, with cases of the same caliber and cubic capacity, but with shoulders of different slopes have failed to show that the shape of the shoulder makes any difference at all.

“Usually the originator of one of these cartridges had no facilities for taking pressures, so depended entirely on the notoriously unreliable method of judging pressures by the appearance of the primer.” 

Now, this was written by a man who’d spent decades using the best pressure-testing equipment then available. So why do so many sources of handloading information, including loading manuals published by major companies long after Hatcher’s Notebook appeared suggest primer appearance as a valid pressure sign? In fact, one such manual was published in the 1960s by a bullet company that purchased pressure-testing equipment but apparently couldn’t figure out how to use it. Instead they depended on primer appearance (and other such seat-of-the-pants signs) to work up their published data, with absolutely no verification of the actual pressures.

Unfortunately, Hatcher did not explain his statement in the Notebook; otherwise millions of handloaders would have quit wasting their time “evaluating” primers. But by reading other sources, including American Rifleman (my collection goes back to the late 1920’s), here are some of the reasons:

The major pressure-indicator is supposedly how much the surface of the primer flattens during firing. This flattening occurs because as pressures build inside the case, the gas not only pushes the bullet down the bore, but pushes backward through the flash-hole against the expended primer, forcing it against the bolt-face/breechblock. The impact flattens the thin cup of the primer. (In fact, the pressure starts the primer moving backward before the bullet starts moving forward, because the powder immediately in front of the primer obviously ignites first.)

There are several reasons flattening doesn’t work reliably as a pressure sign. First, different rifle primers have cups of different thickness. Second, while they’re normally made of the same brass used in cartridge cases, about 70% copper and 30% zinc, the ratio can vary somewhat, just as it does with cartridge cases.  Both the specific brass and thickness of the cup can result in a different amount of flattening.

Also, relative primer flatness can be affected by even a slight amount of headspace, the distance from the rear of the case and bolt face/breechblock. If the pressure’s below about 45,000 PSI, the primer won’t flatten much as its pushed backward in its pocket by expanding powder gas. Plus, unless the chamber’s well-oiled, the relatively low pressure isn’t enough to push the case itself back against the bolt face/breechblock, whether through sliding or stretching. Instead the case sticks to the chamber walls, and the primer remains slightly backed-out of the case.

This is common in the .30-30 Winchester, because the SAAMI maximum average pressure is only 42,000 PSI. The standard SAAMI maximum headspace allowance for just about all centerfire rifle cartridges is .007 inch. This may not sound like a lot, but when a primer backs out that much it’s easy to see.

In higher pressure cartridges, from the .22 Hornet (49,000 PSI) on up to the maximum 65,000 PSI allowed by SAAMI for any rifle round, the pressure does push the case-head against the bolt-face/breechblock–unless headspace is absolutely minimum, often due to neck-sizing after being previously fired in the same chamber. Generally, full-length-sized cases will have a little headspace, and obviously new cases (the kind many handloaders use to start working up loads) may have up to .007 inch.

This results in the primer backing out first, whereupon the rear of the thin cup expands sideways slightly, because it’s no longer supported by the primer pocket. As pressures continue to rise, the case backs over the primer cup, and the slightly expanded brass at the rear of the primer cup is squeezed back into the primer pocket. This results in the primer appearing VERY flat, even at relatively low pressure.

This “false flattening” can also be enhanced by the edge of the primer pocket. Some pockets have considerable radius at the edge, while others are relatively sharp.

Three other commonly advised primer “pressure signs” are a ridge around the firing pin indentation (where the primer brass flows into the pin’s hole in the bolt face/breechblock), a pierced primer (where the firing pin indentation blows out), or a leaking primer (indicated by black residue around the edge of the primer).

Unfortunately, like primer flattening all three can occur because of factors other than excessive pressure. Many rifles have rather loose firing pin holes in the bolt face/breechblock, allowing primer brass to flow into the hole around the tip of the firing pin. A weak firing pin spring can also result in the same “false positive,” and both effects can be enhanced by thinner/weaker primer cups.

A pierced primer can easily occur when the tip of the firing pin is too pointed, and primers can leak around the edges due to brittle or cracked primer cups, especially where the cup is “folded” during forming along the corners. Again, high pressure doesn’t necessarily have anything to do with the piercing or leaking.

The other factor in all this, of course, is most handloaders (like the aforementioned bullet company) have no way to actually measure pressure. As a result, they have no idea whether any of these mystical pressure signs are valid.

The one sure primer pressure sign I’ve encountered during decades of handloading rifle cartridges is a primer that disappears during firing. This is known as a “blown” primer, caused by pressures so high the primer cup disintegrates, because the primer pocket expands so much it’s far larger than the primer. This takes a LOT of pressure, and I’ve only seen it a handful of times, including (more than once) with factory ammo. Only two primers were blown by a mistake on my part, both caused by assuming too much, and I’ve NEVER repeated either mistake again.


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