We usually want the most velocity we can SAFELY get, and here’s all about how to stay safe. Keep reading!

Glen Zediker

I’ve been on the topic of load development — “working up” a load — for the past couple of editions, and, based on the excellent feedback from you all, here’s more. As always, there’s only so much I can write before I have to cut myself off.

I’ve said that velocity is the initial leading indicator of pressure. Velocity, in itself, however, is not a definitive indicator of pressure. I’d like to clarify… The first point is that I am a big believer in establishing a goal for load development, and, for me (and likely most others) that is a velocity. Accuracy is a given! I will never consider a combination that’s not shooting little knots downrange, but accuracy and velocity are not mutually exclusive. I also would never consider a combination that produced very small groups at an unacceptably low velocity, and that’s because I’m shooting (always) beyond 200 yards. The super-accurate low-velocity load gets its bullet shifted that much more in a variable wind, so it’s way on less likely to maintain those small groups.

I want to hit the velocity ballpark I have in mind and that’s why chronograph readings as I’m incrementally increasing the propellant charge are my leading indicator to how close I’m getting. I am also, always, looking for pressure signs on the spent cases — each and every one ejected.

So about those pressure signs…

Primer condition gets first attention.

primer pressure signs
Middle is what I want to see: pretty much a new primer with a nice round dimple in the center. Right, well. Massive pressure! But notice that the primer still shows a radius on the edges and is only a little rougher in appearance, well, aside from the crack…

A primer should have a smoothly dimpled firing pin indention, a shiny appearance, and a visible radius on its edge. If any of those are missing or compromised to varying degrees, there’s your sign… A dull and flattened primer has been abused, as well as one with a pitted or cratered appearance. Clearly, a crack or leak (indicated by black fouling) is way over the limit. After experience, backed up by gauged measurements, you’re liable to find that judging what’s “normal” and “safe” from one rifle can be different from another. I have had individual guns that flattened primers at any point near a safe-maximum charge. And, I’ve had them that just lied. Unfortunately, small-rifle primers don’t show always show pressure signs as reliably as large-rifle primers (structural differences). I’ve had experiences where the primers are all nice and shiny like and then blow out with the next increment. Shame on me for taking it there, and, speaking of: don’t get greedy! That’s one reason a velocity goal is important. Despite what your kindergarten teacher told you, you’re not that special… If you’re reading another 50+ feet per second more than what consensus says you should, better bet you’re over-pressure. “We” went through a lot of that when coated bullets got popular: those changed all the rules for “maximums.”

flattened primer
Here’s flat. My experience has been that large-rifle primers tend to display this indication more so than small. What’s happened is that the primer has flowed quite forcibly to fit the confines of its pocket and the bolt face. It’s also normal for some rifles, but that just means you have to know: pay attention and back off if you see a flattened primer.

The best pressure indicators show at the loading bench.

primer seating
My best “gage” for pressure is seating a primer in a fired and resized case. It’s a feel, gained through comparative experience, but too easy means there was too much pressure.

The reason I suggest (strongly) doing load work-up with new cases is because you then have a baseline. Measure the case head diameter (on the case, not the rim or groove) on the new case and compare it to the fired case. Up to 0.0005 (that’s ten-thousandths) is really high but some say acceptable (not me), and 0.0002-0.0003 is what I’d prefer. Plus, since a new case is at its smallest, meaning it will have a little less capacity than a fired case, you’re getting some assurance that the pressure will likely be a little lower from the same load in subsequent reuses of that case.

All dimensions are at their minimum in a new case. Primer pocket expansion is related to case head expansion. I get (what’s proven to be) a very accurate indication of pressure based on the resistance to seating a primer in that resized case. You have to use a priming tool that gives adequate feedback (meaning low leverage) but if the primer just slips right back in, that load was over-pressure. In a more extreme circumstance, the primer won’t stay seated. Yes. I have seen that. Shame on me, again.

Finally, a new case easily points out the difference between a “pressure ring” and a “sizing line” that can show just above the case head along the case body. A bright ring there indicates excessive stretching (a sizing line comes from the die reducing that area, and is perfectly normal). That “pressure ring” sign is also likely an “improper headspace” sign, but that’s another article.

pressure ring
Here’s a “pressure ring.” This poor old fellah used to be a brand-new Lake City Match case. I suspect there was some issue with this rifle’s headspace, but if you see this bright stretch mark, red flag it! It means the case is going to crack right there next use (called an “insipient head separation”).

Pierced Primers
This is a common malady on AR-platform guns, and especially on the big-chassis versions (SR-25, AR-10, and similar). Pressure both isn’t and is the culprit and the solution. Lemmeesplain: What causes the pierce is a firing pin hole that is too large. It is not the fit of the firing pin tip to the hole! An engineer can explain it, but it has to do with surface area covered by the firing pin hole, and then along with it the surface area of the primer. Simply: the firing pin hole turns into a cookie cutter. A primer pierce creates all manner of ills, including wrecked firing pins, gas flow through the charging handle area (where your face is), and abrasive debris scattered throughout the lower interior, including the trigger parts.

firing pin hole size
Blueprints call for a 0.058-inch diameter firing pin hole on an AR15 bolt. If the hole is too large then primer structural failures (pierces) will, not can, rear up. Too big is anything more than 0.062 inches, and I’ve seen plenty bigger than that. I use machinist’s drill bits to quick-check bolts: 1/16 (0.0625) and #53 (0.0595). If the first fits the hole, find another bolt. If the #53 won’t go, use that bolt with confidence.
pierced primer
Notice that this primer doesn’t really show excessive pressure signs. Just has a hole in it…

Excessive pressure gets blamed for a pierce but what’s really going on there is that it’s not certain that amount of pressure would be judged as “excessive.” It’s just gotten high enough to bring on this result. So, yes, lightening the load will stop the piercing, but, in my experience and that of many others, the pierces can start happening before reaching what most might agree on is a max load. I say that because “we” are all shooting about the same bullet/primer/case/propellant combinations in NRA High Power Rifle (with respect to Service Rifle division AR15s, for instance). Seeing pierced primers before hitting the proximity of competitive velocities points to “something else,” and that is the firing pin hole.

In a truly over-pressure load, the primer can crack or blow slap out, but it won’t pierce.

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.

19 thoughts on “RELOADERS CORNER: Pressure Signs”

  1. Don’t agree with head diameter because name brand brass manufactures have been putting out new cases under limits on size must be trying to get longer runs on header dies or to use less brass in making cases.
    Quality of new brass has gone down with the high demand for it.

  2. Had an old 30-06 Enfield that started to show “pressure ring”. Took it to my gunsmith and was told the chamber was “stretched” and would have to have the barrel replaced. Wen’t home and thought about it, and decided since the gun was almost 100 years old it was time to retire it.

  3. You forgot to mention that different cases( w/w, lapua for example) have different case volume. Always stay with the same case manf. when working up a load.

  4. Keep articles like this on coming thanks!!! Even as an experienced Reloader, with over 7,700 reloads for my Bolt Guns & Revolver, using Lee’s Hand Press (not a Bench Press) and Lee Loader Kits, literally making one round at a time, this is an ‘Important’ issue to stay on top of! Great Job!
    In Liberty,

  5. Excellent ! Wish I had read an article like this before I started reloading many years ago. I hope ALL you Newbies take heed to the above article. Memorize each picture…each warning sign and work up slowly.

  6. Article highlights some good points. For us English-speakers, translate “visible radius on its edge.”

  7. It gets a little confusing, just looking at primers for supposedly standard signs. For example, there is what you see in a .223 Rem, but that may be somewhat different with a 30-06, and maybe again different between bolt action and semi-auto. Then, there are differences in primers. For example, Federal uses a softer metal in some of its primers than other manufacturers. Also, Remington recommends its bench rest small rifle primers in .223 Rem and other higher powered loads, because its regular small rifle primers sometimes don’t handle that pressure. So, I would not depend on primer appearance alone to tell me whether my pressures are right. I’d go much more with reloading manual specifications, measured bullet velocity with a range of loads in the specified load allowances, the approximate powder burn speed in that context and the effect of slightly more or less on the velocity, and then take a look at the primer for good measure.

    I’m not sure why incipient case separation is mentioned in this article, because it really does seem a separate subject altogether. The brass of a cartridge, other than the head and the primer alone, is simply not a pressure holding mechanism. The chamber does the job of containing all of the high pressure in front of the head, and for all intents and purposes the whole forward part of the shell could as well be plastic or paper (as is demonstrated by most shotgun shells and some experimental cartridges in rifles, etc.), as long as the bullet is held securely in position. Incipient case separation is caused by excess head space exclusively, as far as I know. First, the brass case is pushed fully forward by impact of the firing pin. It then expands under even light load pressure and squeezes tight against the chamber walls. Next, the head is pushed backward into any excess gap between the head and the bolt face. This causes the shell to stretch at the thin wall area just above the head, behind where thin brass is pressed against the chamber walls and in front of the head, which is not so tightly squeexed against the walls.

    A break in the shell at that location is deemed by some to be a certain disaster, a sudden release of pressure toward the back of the firearm, but again, the back of the chamber remains pretty well plugged by the head, and the thin wall at the split is not designed to restrain pressure beyond being just the rim of the cup formed by the head, which continues to plug the back of the chamber.

    Even if an incipient case failure is not highly dangerous, it is far from desirable, and any signs of it happening should be an alert. A problem that can easily arise is that the top of the shell will remain in the chamber after the extractor pulls out the separated head, and that upper case can be a pain to dig out. It prevents loading of the next cartridge, and the time and possibly tools needed to clear the problem can lead to substantial down time for the firearm.

    Meanwhile, firing a gun with overly pressured rounds is one way to gradually develop excess headspace.

    1. Glen, Great article! Bill, Great retort! I’m guessing both of you are in the industry or have been doing this for a long time.

  8. Should look at the case size may show some stretch because military guns usually have larger chambers to insure it to go into battery. I have the same type of rifle and they are one of the strongest 30-06 ever made.
    Did gun smith make a chamber cast to measure it with if he didn’t then see another gun smith .

  9. I stopped reading when he demonstrated his lack of knowledge about dimensions.

    “Up to .0005 (that’s ten thousandths)”

    Uhhhhh what?????

    1. Kevin, .5 is 5 tenths, .05 is 5 hundreths, .005 is 5 thousandths, and .0005 is 5 ten thousandths. He was just stating that it was the ten thousandths place.

  10. Good article as always! You mention the black fouling commonly seen around the primer indicating over pressure. Would you agree the same can be seen from a load that is under charged? I am working on a 30/06 load and I was down on the very low end of charge data and saw this along with primers that had backed out of their pocket. This led me to believe that the charge was too low because the primer didn’t reseat itself after firing. Would you agree?

  11. Hey Kevin, Mr. Zediker is reminding everyone that the “5” in 0.0005″ is in the Ten-Thousandth Scale, not Thousandth Scale.

    I’d encourage everyone to use the exact same spot on the Case Head when measuring Case Head Expansion(CHE). And it is a good idea to do at least 3 Cases at each Load level because the Case Head rarely expands concentrically (meaning not hardly at all). If you are working your Load “Up” then there should be no real surprises.

    Pressure Ring Expansion(PRE) is a bit different in that a person should rotate the Case to locate the widest diameter ahead of the groove before firing. Then rotate the Case again to locate the widest diameter after firing to see how much it has expanded. And that widest measurement is rarely in the same position. It is because of differences in the thickness at that spot during manufacturing as well as variations in the alloys. But, the expansion is more concentric for PRE.

    It is important to understand these measurements must be taken with a 0.0001″ capable micrometer. A caliper IS NOT accurate enough.

    Mr. Zediker, Nice article!

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