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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 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 1960's 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 thickness and the specific brass 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 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 much, 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.

Nice writeup, JB. Maybe you've made headway in killing off the notion that primer appearance can be read as a pressure sign. Hard to slay some of the old lore that persists, though.

Good information. I should tape it to the loading bench.

The one point you quoted from Hatcher that I am not clear on is that shoulder shape has no affect on pressure. Maybe in regards to high or low pressure but I thought and you have stated that the 30 degree shoulder appears to produce more uniform pressure according to the ballistics labs.

The same on case shape, no difference, but then some claim a shorter powder column is more uniform.

Also how does this relate to bushed firing pin holes and small rifle primers showing (producing?) less pressure? Lots of questions left for Gun Gack III.

Given the equipment of the time Hatcher could not report on shot to shot variation with the precision that a lab can today, and I wouldn't think he could have looked at a time-pressure curve for an individual shot at all.

The average pressure produced by two cases of the same capacity will be the same, but the other statistics may vary.

Your average Joe would trust a credentialed army general, especially one who worked in the Ordnance Corps. Much of it can be attributed to the time. That included a lack of test equipment and new low and high explosive compounds developments that were not fully understood by users. And of course, when an officer, especially a senior one, renders his opinion, he is rarely questioned.

What were General Hatcher's credentials? I know he graduated from Annapolis, but I wonder what he studied? Was he an engineer? You see this, but it really doesn't say anything.

"Julian Sommerville Hatcher was a noted firearms expert and author of the early twentieth century. He is credited with several technical books and articles relating to military firearms, ballistics, and auto loading weapons."

Bushed firing pin holes reduce "blanking" of the primer cup material into the hole. They do not change the pressure at all.

John, good write-up. I have a question. Many years ago, someone (can’t remember who) was selling a “strain gauge” for measuring chamber pressures. What ever happened to this product. Not reliable, too expensive, etc.? Thanks! memtb

Google "pressure trace."

See e.g. the thread on this board #942440 - 07/27/06 RSI Pressure Trace and Chrony 5 pages of running on as is typical on this board. Tooling is still available. Reliable in the sense of consistent measurements and affordable for limited use. Discussion by Ken Howell, RockyRaab, John Barsness, Denton and others including more reliable like those cited and others.

IMHO the major issue is calibration so that strain gage readings can be crossed to psi measurements. Secondary issues associated with barrel vibrations and maybe resonance have given some confusing results with multiple pressure peaks.

As I read it Ken Oehler suggests the set up would be better with a proof channel. That is there are as many questions as answers and this has kept the system from sweeping the field.

Very article, MD. I particularly liked the last sentence..... Been there done that as well. At least I discovered why mauser extractors are so popular. In both cases, one was a tang safety Ruger, and the other a 98 mauser, the action could extract the cases without pounding the bolt with a mallet. E

I have a Remington 7 mag that will flatten primers ( federal) even with starting loads. They chronograph normally and do not show any other pressure signs. I assume its because of soft primers.

Interesting read.

Something I have noticed lately is in seating primers. I have always swiped my thumb across the primer after seating and 'felt' for them to be properly seated. But I was having a few rounds that would chamber slightly stiffer than normal. I believe that was caused by the primer not being completely seated. I have since been seating the primer then laying the case (primer down) on the back of my dial caliper (nice flat surface) to verify the primer is fully seated. I have been surprised at how often a primer is not fully seated and will rock back and forth on the caliper. This may also contribute to the primer flattening?

I have a problem with that statement. The pressure inside the case is equal in all directions.

I have a Remington M700 BDL in 30-06 that will give a cratered primer with just firing an empty primes case. I bought the rifle NIB from a department store that was going out of the sporting goods portion of their sales. On close examination with a magnifying glass, one can see where the firing pin hole has a slight chamfer. Actually you can see it without the glass but it just makes seeing it in better detail. I've run some pretty stiff loads though that rifle and other than that cratered primer there's been no problem.
Paul B.

Comments so far:

MartinCreek,
In general, Federal primers often do flatten more than CCI, Remington and Winchester primers, because their cups can be thinner. In fact, I've owned both a rifle and a revolver that would go bang consistently with Federal primers, because the cups were more easily dented by the firing pin. It turned out the mainspring on the rifle was going bad, and eventually it wouldn't make any primer go bang. The revolver was a Ruger Redhawk with a "spring kit" installed, which made the trigger pull a lot better but also made Federal primers necessary.

Centershot,
Protruding primers just might contribute to flattening! (Now I might have to run some experiments.)

Muskegman,
Yes, the "pressure inside the case is equal in all directions." But that is true of the eventual maximum pressure. Initially, pressure always finds the path of least resistance.

In this instance that's through the flash-hole, rather than at the other end of the case where the bullet starts moving. Even in very small calibers, the bullet is not only heavier than the primer, but usually held far tighter by the case neck, because of more contact with the neck and, sometimes, crimping. (It takes a lot more force to seat a bullet than a primer.)

Plus, the normal "high" primer of fired .30-30 cases indicates the primer was backed out the initially relatively low pressure.

What a coincidence...This week, I brought "Hatchers Notebook" as reading material for my long flights. Rereading this book never gets old. New things stick in my brain each time.


Ken Oehler offered his M43 PBL, which to my knowledge was one of the earliest consumer oriented pressure testing units. Around the same time Steve Faber also produced one. Both are strain based and preceded RSI's Pressure Trace.

OH but MD...............whatever happened to "all other things being equal?" when it comes to things you DON'T push.

LIKE the 4-1 "rule" (very weak guesstimate that originated in the 60s by Homer Prowley with the ALSO antiquated "Prowley Computer" where it DID actually give SOME information because IT WAS APPLIED TO CAPACITY INCREASES MADE IN THE CHAMBER OF THE SAME RIFLE, WITH THE SAME BARREL, WITH THE SAME BULLET JUMP, SAME SEATING DEPTH, SAME EVERYTHING......EVEN THE SAME POWDER")

Back THEN........it was "about", "kind of close", "in the area of"........this much increase in velocity. Comparing rifle to rifle? Cartridge to cartridge" Regardless of ALL OTHER FACTORS that make UP a rifle.......is ludicrous.
Until EVERY rifle shoots the velocity given IN the compared load data used in the aborted version of the "4-1 rule"........it is entirely, absolutely, and without a doubt.............POINTLESS.
Going from factory barrel to a decent custom barrel can give you up to 100 fps........or more, alone, depending MUCH upon the very, very, very small changes/differences in bore/rifling diameters. Being BETTER barrels as far as accuracy goes is NOT the only differences.

Anyone doubting this please compare Nosler's load data pages to the Hodgdon reloading data site. Compare the same powder, same bullet (you have to look some), same barrel length and see how often (almost always) the Nosler data is way more (often 100 fps plus) than the Hodgdon data. They look at the name NEXT TO the barrel length on the Nosler site. Nope..........."all things are not equal".

Comparing two rifles...........THEY ARE NEVER EQUAL. There does not exist a "rule of thumb" on how high or low the velocity/pressure/accuracy/anything else compares. Trying to create one is a waste of your time and thought processes. Your rifle, your brass, your powder and primers, your efforts as a hand loader, make every combination an entity of it's own right. No one, no data, no arbitrary "maximum pressure" applies. It's safe in your combination........or not.


The information given here in the OP is OLD...........OUTDATED..........and NOT comparing primer to primer fired ALL OTHER THINGS BEING AS EQUAL AS POSSIBLE..........makes it all worthless..

No, all things cannot be equal.........ever, but handloaders SHOOT FOR such for low ES and low SD, (as consistent as possible) and DO use primers as a half decent high pressure sign.............because it is when the person loading makes all efforts to properly load rounds (as close to identical as he/she is able) , firing them in the same chamber (that they KNOW is clean and dry) .
Every long range competition shooter KNOWS the "weak link" in high pressure loads............is the soft (by comparison because it is not SOLID metal and not the same thickness as case base material) and it is PRESSURE FIT into the case. What gets pushed IN can get pushed out.......if there is "room" (bad sizing) but if not..........the primer TRYING to push out enough to deform it......IS HIGH PRESSURE.

Many studies have been done by competition long range shooters on primer hardness and size for that very reason.
Proper primer fit, proper case to chamber fit, headspace etc etc is a "surely you are doing that" kind of thing because experienced hand loaders know this is the main key to consistency along with the same components. They are not mentioned because it would be an insult to the other person asking them if they knew what the H they were doing...........just not saying it straight out.

What also gives "high pressure signs" in loads/chambers/ etc concerning improper care or loading............is moot and does NOT take away from the fact that ...............if you get primer deformation in loads you know for a fact are as equal as possible........and then you back off a half grain and you DON'T have primer deformation.......................IT WAS HIGH PRESSURE. (Talk about a no-brainer).

*******A short word (oops.......too late for that,eh? LOL) to those here that can think for themselves. READ the pages before and after SAAMI "Maximum Average (repeat AVERAGE!) Pressure" charts. I'm not going to tell you what they say. I have complete faith that those who understand the English language and will take the time to read through it all..........will see some BLATANT differences in what is tossed about. Special attenton to WHO SAAMI IS, What they can and cannot do.......and do not set!, How testing is done. What factors can cause errors in their tests (any anyone else's!!).

On pressure..........look ESPECIALLY for a statement on "97.5 percent of the service rounds will not exceed WHICH Maximum? and last but oh so far from the least.......Look for the STANDARD OF DEVIATION for their MAP findings. AND ALL THE OTHER PRESSURES within their report!.

Take the "Maximum Average Pressure" out of context and deciding what it is or is not, is like reading the 10 Commandments ( Exodus 20 KJV version) and seeing "Thou Shalt Not Kill" ..............and deciding to sell all your firearms because of what that "means".

Forget what you've been told.......read it all and read it again. Then give thought to this....... When you understand what the REAL maximum pressures THEY PREDICT are within FACTORY ammo, recall how often you have seen a rounded or flattened primer using them (it happens.........rarely) ...........then rethink what pressures WE are at when we sneak up on max (FOR OUR RIFLE) but back off when we get to pressure signs. Especially when you realize where your pressure MIGHT be when you see for yourselves the standard of deviation on chamber pressure ESTIMATES. And yes........all means of measuring chamber pressure have one. And yes............the Piezoelectric testing is considered one of the most accurate.
That's why SAAMI uses it. (I'm sure SOMEONE will huff and puff about the "strain gauges" now........oh well. YMMV?)

I'm not attempting to get people to load hotter............I'm trying to get people to think and decide for themselves. (Yes MD we KNOW........."You load to velocity"...........gimme a break.)

MODERATORS! While I would love to have a decent conversation on this, I will not return to reply on this post. The blatant disregard for controlling insulting, idiotic ramblings on this site make it of no interest to me ..............what-so-ever.

Hint.................


My apologies to those having a true interest in finding out for themselves (aka thinking) for my poor writing abilities and the mis-spelled words. I don't have multiple people proof-reading my work, plus it's just past 2AM. I only hope I've reached............at least a few.
YOU GOT THIS! You don't need MY or "anyone else's" help.

Have at it bashers. I can insure you that, I for one, will not waste my time reading them.

Oh.........MD? YOU owe Homer Prowley a public apology for aborting something he offered us into the lunacy you've stated as any kind of useful information. I mean............come on. Buy five rifles, all in the SAME caliber and they won't often shoot within a 50 fps span of each other. Guesstimating velocities CARTRIDGE to cartridge ALSO in differing rifles?..............zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.

God Bless. Hunt safe.
Steve

Am I the only one looking at this post thinking, "What the hell is all this pointless rambling"? I believe that I'll take John's side in almost every case (although I'm still not a fan of NBT bullets, but experience is limited to .25 caliber), due to his extensive testing, research, and thoughtful analysis. He also has no problem telling us when he's discovered, through trial-and-error or learning from other professionals in the firearms/ammunition/reloading industries, that what he has thought to be true isn't necessarily so. Guess he never thought of himself as a scientist, but he does use scientific principles to arrive at his conclusions. So, Steve....

Agree. I found myself wondering what Steve's point is also.

Dunno either, and can only guess at what Steve's point is about Homer Powley and the 4-to-1 Rule.

For those who aren't familiar with it, the 4-to-1 Rule is that any increase or decrease in the powder capacity of rifle cartridges of the same caliber results in about 1/4 that amount of potential velocity, with the same bullet at the same basic pressure. One example I've used frequently is the .300 RUM and .308 Winchester: The .300 RUM has just about twice the powder capacity as the .308, but does NOT produce twice as much velocity--a 100% increase. Instead the velocity increase, with the same bullet at the same pressure, is about about 25%, which is 1/4 of 100%. This rule does not apply to the SAME powder, but the powders producing the highest velocities in each cartridge.

I am quite familiar with Homer Powley's slide-rule calculator, having purchased one around 40 years ago--and still have it in my bookshelves, packaged in a manila envelope with all the accompanying literature. Can't figure out what the 4-to-1 Rule has to do with any of Powley's basic rules, except that it's another phenomenon derived by crunching the numbers from LOTS of pressure-tested data.

The only similar rule I can remember from Powley is that pressure with single-based powders increases at twice the rate of velocity--which might be called the 2-to-1 Rule. But that was derived from copper-crusher pressure data, not the piezo-electronic data used by the best pressure-labs today, which shows the velocity-pressure relationship isn't quite that simple, even with single-based powders. It varies even more with double-based powders.

Steve also seems to be arguing that pressure-judging by primer appearance works very well. Would love to hear if he can supply some data. I seem to remember Denton mentioning that he'd tried it, and found no correlation.