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Originally Posted by memtb
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


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One point I think Steve was making, which appears to be true in my 40 years of handloading, is that the pros in competitive matches, using the same rifle and components, have found that reducing a load does show up not only in less velocity but less primer flattening. I generally agree with that IN THE SAME RIFLE, USING THE SAME COMPONENTS.

Bob
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Bob,

Sure, why wouldn't it?

But that still doesn't mean that relative primer flattening is a reliable means of estimating actual pressure.


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Might also mention that I've done a little more primer-appearance testing involving new brass since posting the article. New brass generally has a slight amount of headspace, ecpeacially in factory chambers, though not more than the +/- .007" allowed by SAAMI specs.

First, I noticed that a lot of the brass from once-fired factory ammo had noticeably flattened primers, even though all of it never showed any other sign of excessive pressures, including velocity. This is due to the same reason I mentioned in my post: New brass backing up over a primer that's backed out slightly, then expanded, before the case expands enough to fill the chamber.

Also, with new primed brass, which isn't loaded with powder or bullet, the primer usually backs out a little from the case-head when the empty case is fired. Apparently the "rocket effect" of just the priming compound going off is enough to push the primer backward in the pocket.


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Nicely put—good article.

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I will attempt to add something useful.

Mule deer mentioned the movement of the primer in the case and while I know that to be true, I've never went about proving it until this morning when I was proving (to myself) that small pistol and small rifle primers are the same dimensionally.

ANYWAYS, I found this large rifle (probably a Federal #210) primer in the catch cup of my Bonanza CoAx. It shows (to me at least) definite movement and "riveting" from the case head (which must be harder brass than the primer cup)

[Linked Image]

If the image is not clear enough, I can retake the picture with a "real" camera and repost it.

Also: Would not the brass in primer cups, while of the same composition, be "softer" due to less work hardening during manufacture?

RJ


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I had (2) consecutive primers blow outta the same box of factory Horn 28N eldx ammo. The double pressure spikes caused the firing pin to blowout the back of its housing and hang the firing pin on the lip edge after it smacked my right thumb.

Hornady asked for the remaining 18 rounds to be sent back for testing to determine if this box got an extra powder dump.

I’m waiting on the results...As someone already mentioned, this can be a common occurrence with factory ammo. 😎

[Linked Image]


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Well, for Pete's sake, of course you had trouble...it's a 700. Can you spell Mauser? Seriously glad you weren't badly injured.


Well this is a fine pickle we're in, should'a listened to Joe McCarthy and George Orwell I guess.
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Me too actually....I’ve blown a primer on occasion but (2) in a row, never....I sent the rifle back to the builder for a check up.

They buffed the firing pin but otherwise said its gtg...Mowser for the win. 😬😎


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I find John provides info when it is needed. I did need to read this.
I once accidentally used CCI pistol primers while developing loads in a .270- ignition was poor( of course) and the primer strike perforated the primer.
I had purchased 4 and within the taped package , was the no good pistola primers. I seated a few( mistakenly) and then decided to give them a go.
A few ignited the 4350, a few didn't and perforated the primer.
CCI pistol primers were packaged in blue and white slightly different than LR primers.
Anyways, I digress, John keep doing what you do.
Love you stuff

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Originally Posted by recoiljunky
I will attempt to add something useful.

Mule deer mentioned the movement of the primer in the case and while I know that to be true, I've never went about proving it until this morning when I was proving (to myself) that small pistol and small rifle primers are the same dimensionally.

ANYWAYS, I found this large rifle (probably a Federal #210) primer in the catch cup of my Bonanza CoAx. It shows (to me at least) definite movement and "riveting" from the case head (which must be harder brass than the primer cup)

[Linked Image from i.postimg.cc]

If the image is not clear enough, I can retake the picture with a "real" camera and repost it.

Also: Would not the brass in primer cups, while of the same composition, be "softer" due to less work hardening during manufacture?

RJ



Mule deer???


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Huh, too good to answer my questions are we John?

RJ


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Didn't realize you were asking a question. It seemed like a statement.


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Looking for your input, agree/disagree etc.

RJ


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That looks exactly like the "riveting" (or whatever you want to call it) that happens when a primer backs out of the case a little, due to a slight amount of what might be called "extra" headspace--though it really isn't, at least according to SAAMI.

As mentioned in the article, SAAMI headspace specs usually allow for about .007" of variation in chamber/ammo dimensions. At typical modern bolt-action pressures, this often results in "flattening" of primers, due to exactly what you posted: The primer backs out a little before max pressure is reached, with the rear of primer expanding due to not being supported by the primer pocket.

However, the case head is not necessarily "harder" brass than the primer cup, just thicker.


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Gotcha, the reason I wanted "to know" was there is some belief that there is a difference in material between pistol and rifle primers and that pistol primers can't withstand the pressures "generated" by rifles. OK, maybe, but small rifle and small pistol primers can be interchanged, but due to dimensional differences large pistol (shorter) and large rifle cannot.

Yes?

RJ


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Take cup thickness into account, not just height.

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That's what got me thinking . . . Very dangerous . . . So I started measuring heights, thicknesses etc as best I could and with all the Federal and CCI primers I measured, cup thickness between both brand for all types of primers (large and small rifle, large and small pistol including magnums of all four types) cup/wall thicknesses were the same. The only difference being height between large pistol and large rifle.

RJ


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Headspace determines primer appearance. The primer always comes out first before being reseated by brass expansion. Look at a straight wall revolver case. Headspace is not at the shoulders since there is none. It is the distance from the case head to the recoil plate. Every single .44 mag can have a dead flat primer well within pressure limits. The force from the flash hole will send a hammer back to near full cock. The squirt from that hole can be horrendous well before peak. To rivet a primer as shown means headspace was excessive by full length sizing too much. It has nothing to do with final pressures.
Fed is supposed to be weaker but I have taken Fed 150 primers to near 60,000 # in the .454 with cut down .460 brass without a flat primer. I tested the 150 to the .500 S&W but the 155 was more accurate and my go to from the .475 up. The 150 is the only for the .44 mag.
Weatherby had ignition problems at the start. Fed was the first to make a mag LR primer that brought the Weatherby to life.

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Originally Posted by CZ550
One point I think Steve was making, which appears to be true in my 40 years of handloading, is that the pros in competitive matches, using the same rifle and components, have found that reducing a load does show up not only in less velocity but less primer flattening. I generally agree with that IN THE SAME RIFLE, USING THE SAME COMPONENTS.

Bob
www.bigbores.ca


I am far from being any pro, or an engineer for matter. But my observations support this statement.

For several years, based on what I learned reading commonly available literature like my loading manuals, and before the advent of internet message boards, I did a lot of experimenting with several rifles. Most notably a pushfeed Win 670 in 30-06, and also a rebarreled Venezuelan mauser in 30-06 AI, and eventually a Win 70 classic in 264.

My efforts involved brass fire formed in the rifle, resized in FL dies in a manner which only 75% of the neck was sized, and primed with CCI 250 primers.

Then I painstakingly charted charge weight vs velocity graphs for several powders and several bullets over the course of about five years. (note: my experiments exceeded the life expectancy of the 264, and it had to be rebarreled) Some loads were pushed to the point of expanded primer pockets where they would no longer hold a primer after three or four loadings. And I experienced a couple of "pressure excursions" which expanded the case head to the point it would not fit into the RCBS shell holder on my press. "Pressure excursions" being with 140 gr partitions in the 264 loaded over early production RL25. A load which shot under 3100 fps in Nov, scared the hell out of me in July.

Anyway enough background, to observations.

Loss of primer radius definitely correlates to increasing velocity, increasing powder charge, and obviously increasing pressure. The phenomenon is observable and repeatable. I feel it can be used by a very careful observer to indicate increasing pressure values. But every other facet of the load must be held identical.

But the question is, what actual pressure value does the flattened primer represent?
Hell, I have no idea! That is what the chronograph is for.

And how do you define or measure the degree of primer radius loss?

So yes, it is a tool which can be used by the very careful, dedicated, and experienced handloader. But not one to be recommended to the novice as a be all do all safety check.

Now as far as CHE. As Denton told us a decade ago, there is too much noise in the system to get any reliable information. I meticulously measured hundreds of cases before and after firing. The head expansion numbers were so random that they were meaningless.





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