More about submarines

[K Frame]

Good article on pressure testing methods from Buns & Whammo.

https://www.gunsandammo.com/editorial/ammunition-pressure-testing/458750#:~:text=The%20copper%20crusher%20is%20placed%20on%20top%20of,copper%20crusher%20cylinder%20between%20the%20piston%20and%20anvil.

[WLJ]

"If you'd care to go back and forth, the formula is PSI=-17,902+1.516*CUP"

I've seen that before, and I've never understood where it came from. It was never used in the industry because while it might work as a conversion method for one cartridge it can provide WILDLY different numbers for other cartridges.

Yeah, the numbers it comes up with are basically meaningless yet I keep seeing it on the internet. There is no direct reliable conversion formula for converting CUP to PSI and vice-versa
Try it for 7.62x51, the result is way off.

[dogmush]

"If you'd care to go back and forth, the formula is PSI=-17,902+1.516*CUP"

I've seen that before, and I've never understood where it came from. It was never used in the industry because while it might work as a conversion method for one cartridge it can provide WILDLY different numbers for other cartridges.

I would argue that if that's true (and I'm not saying it's not true) then it's evidence that one of the measuring methods is wildly inaccurate.  If you were accurately and repeatably measuring the pressure, than you could convert it to another unit reliably.

Yeah, the numbers it comes up with are basically meaningless yet I keep seeing it on the internet. There is no direct reliable conversion formula for converting CUP to PSI and vice-versa
Try it for 7.62x51, the result is way off.

You sure?  When I google "7.62x51 pressure in CUP" I get:

The maximum pressure for 7.62×51 set by NATO is 50,000 CUP. There's no direct conversion from CUP to PSI, but testing has shown that maximum loads of 7.62 NATO generate about 58,000 PSI.

50,000*1.516-17,902=57,898

[shrug] seems pretty close to me.

[WLJ]

Every PSI rating for 7.62x51 when you filter out the mistaken 50k ones  I've seen states 60,000. Not a huge different but it's still off.

[K Frame]

As I said.... that conversion works for one cartridge...

Oddly enough, it works for .308/7.62...

My theory is that someone came up with it in the 1960s or 1970s to show the relation between CUP and early PSI tests for .308/7.62 and it was never intended to be a universal conversion.

Try it with other cartridges, especially some of the magnums or some of the low-pressure rounds. Have fun!


What's really telling about that "formula" though, is where you DON'T find it.

You don't find it in industry publications.

You don't find it in reloading manuals for the home user.

You don't find it anywhere, really, where there's any level of actual fidelity that it would be useful.

[dogmush]

OK, Sure whatever.  It's not my formula, I had just read it.

But just to be clear:

Your claim is that there are two different methods of measuring Chamber Pressure, that spit out two different units of measure, and those units have no correlation to each other?

If that is the case, one (or both) of those measuring methods isn't working.

[WLJ]

If that is the case, one (or both) of those measuring methods isn't working.

Yep, guess which one.

Until sometime in the 1960s, the common method of testing cartridge pressure involved fitting copper plugs into pistons in a test barrel and measuring how much pressure crushed the plug. The measurements produced this way are expressed in Copper Units of Pressure, or CUP. It was common practice to use Pounds per Square Inch interchangeably with CUP--until piezoelectric transducers started to be used instead of the copper plugs. Piezo pressure measurements are much more accurate than copper crushers, and showed that there is no real relationship between CUP and PSI (beyond the fact that they both get higher as pressure goes up). Sometimes, they're the same. Most of the time, CUP is lower than PSI. As a rule, the behavior of copper under pressure is so quirky that there's no reliable way of translating CUP to PSI mathematically; you just have to measure both ways.

Try that formula on 30C.
Check out the chart and note how 30C is the same number in CUP and PSI when 308 isn't. Some cartridges tested out that way or close to it while other are way different.

30 Carbine 40k CUP 40K PSI
7.62x51 50K 60k

 .308 Winchester vs. 7.62x51mm NATO
http://how-i-did-it.org/762vs308/chamber.html

[K Frame]

"Your claim is that there are two different methods of measuring Chamber Pressure, that spit out two different units of measure... ?"

That's not a claim, that's a fact.

And, in fact, there are more than two methods of measuring chamber pressure. Read the article I linked.



"...those units have no correlation to each other?"

Read again.

I've repeatedly said that there's no formula that will reliably convert one to the other across the wide spectrum of cartridges available.

With some of the large bore magnums that "formula" will give you numbers over 100,000 PSI.

For some of the lower pressure, older rounds you'll get a NEGATIVE conversion number. I guess that means when you pull the trigger the bullet will actually move backwards in the case?

In other words, the "formula" has no fidelity and no direct correlation in a manner that is actually useful to, well, anyone.


If this formula actually works and is actually useful.... again I ask... why isn't/wasn't it used in the industry? You know, by the very people who measure, and publish, information of this type for a living.

SAAMI? Olin/Winchester? Hodgdon? Du Pont? Nope. None of them ever used it.

That should tell you everything you need to know.

[dogmush]

"...those units have no correlation to each other?"

Read again.

I've repeatedly said that there's no formula that will reliably convert one to the other across the wide spectrum of cartridges available.

With some of the large bore magnums that "formula" will give you numbers over 100,000 PSI.

For some of the lower pressure, older rounds you'll get a NEGATIVE conversion number. I guess that means when you pull the trigger the bullet will actually move backwards in the case?

In other words, the "formula" has no fidelity and no direct correlation in a manner that is actually useful to, well, anyone.


If this formula actually works and is actually useful.... again I ask... why isn't/wasn't it used in the industry? You know, by the very people who measure, and publish, information of this type for a living.

SAAMI? Olin/Winchester? Hodgdon? Du Pont? Nope. None of them ever used it.

That should tell you everything you need to know.

Right,  So the units of pressure measurement have no correlation to each other.  That's what I agreed to two posts up.

If the units of measurement are not related to each other, then they are either not measuring the same thing, or one or both measurement techniques is inaccurate.  As far as I know they both claim to measure chamber pressure, so either one test doesn't measure that, or doesn't measure it accurately.

[K Frame]

Once again, try reading some of the information on CUP/LUP testing to understand how, and why, it was industry standard for decades before it was replaced.

CUP/LUP measurements worked perfectly well to determine chamber pressure. The methodology showed perfectly useful fidelity when compared against itself.

The fact that it's an obsolete measurement technology today doesn't invalidate the results that it provided.

[WLJ]

It was accurate enough to give them a close enough ballpark idea how strong a chamber/barrel/locking mech/etc... needed to be since most guns, mil rifles in particular, are usually way overbuilt to handle much higher pressures anyway for safely reasons. IIRC proof pressure for 7.62NATO is 75,000 psi per a NATO doc on the matter I came across.

[dogmush]

It was accurate enough to give them a close enough ballpark idea how strong a chamber/barrel/locking mech/etc... needed to be since most guns, mil rifles in particular, are usually way overbuilt to handle much higher pressures anyway for safely reasons. IIRC proof pressure for 7.62NATO is 75,000 psi per a NATO doc on the matter I came across.

Sure,  I'll buy that.  It makes sense, and as you say rifles are often overbuilt.  And certainly Black Powder loads were operating at lower easier to contain pressures anyways.

I am chuckling a little that you described "accurate enough" as "basically meaningless" when compared to an empirical measurement earlier in the thread.

[WLJ]

I am chuckling a little that you described "accurate enough" as "basically meaningless" when compared to an empirical measurement earlier in the thread.

Meaningless may have been the wrong word because it implies totally useless. that's on me.

But it's relative anyway, it was accurate enough for what they needed to know to design and build the guns they were then. Gave them a base point and overbuild from there.
In a modern, we have far better and accurate ways of doing that now sense, no. Modern engineers/designers would be horrified if they found out a test method they were using today was that inconsistent.


Edit:Made a clarification

[K Frame]

It was accurate enough to give them a close enough ballpark idea how strong a chamber/barrel/locking mech/etc... needed to be since most guns, mil rifles in particular, are usually way overbuilt to handle much higher pressures anyway for safely reasons. IIRC proof pressure for 7.62NATO is 75,000 psi per a NATO doc on the matter I came across.

Ding ding ding.

[RoadKingLarry]

Meh, just keep adding powder till it blows up your gun then back off 10% for the replacement gun.

[K Frame]

Meh, just keep adding powder till it blows up your gun then back off 10% for the replacement gun.

Welcome to the European proof process.

[WLJ]

Welcome to the European proof process.

Yep, that is actually a thing

[K Frame]

It actually works for a lot of things...

This one made my Dad laugh hard when it first came out. But then again he was a highway engineer and dealt with a LOT of bridges...

[230RN]

Meh, just keep adding powder till it blows up your gun then back off 10% for the replacement gun.

^

Hatcher has a comment about that in trying to destroy an M! Garand.  They got up to "120.000 'PSI'," when cracked left bolt lugs began to appear.   A  gun with a cracked lug still functioned normally.  (They did get cracked rear receivers when using some of the grenade launching cartridges, but that was a different failure mode: the bolt slammed back too rapidly for the rear wall of the receiver.)

[K Frame]

Someone tried that with the Japanese Type 38 action, as well...

Ackley, I believe.

Filled a 6.5mm case full of various pistol powders and wasn't able to harm the action.

[230RN]

Not to strum the same chord, but in that same chapter on the strength of actions, Hatcher notes that the Jap action, along with several others (.30-30, etc.)  have the cartridge case surrounded by the receiver, including most rimmed cartridges.  Garand took great pains to reduce the weaknesses caused by the extractor cutouts, where the entire pressure of the cartridge is contained by brass only.

Terry, 230RN

[Northwoods]

Someone tried that with the Japanese Type 38 action, as well...

Ackley, I believe.

Filled a 6.5mm case full of various pistol powders and wasn't able to harm the action.

I’ve heard of people jamming a .30-06 into an Arisaka and firing it.  Bullet got swagged to .264 and case was fire formed to 6.5jap.  Rifle was supposedly fine afterwards.

[HankB]

In Hatcher's Textbook of Pistols and Revolvers, he describes a pressure gauge using copper crusher cylinders, but refers to the crusher's pressure measurements in units of PSI. I think the term CUP was introduced later to distinguish between pressure measurements derived from copper crushers and those newfangled piezoelectric transducers. (SAAMI and CIP pressure measurement methods differ, too.)  SAAMI.org goes into some detail on crusher and transducer measurement standards.

And as already pointed out, there is no universally correct formula known to reliably and accurately convert CUP and PSI.

This has led to some odd standards - for example, SAAMI.org  lists MAP for .357 Mag as 35,000 PSI . . . but 45,000 CUP is also acceptable. Some years back one of the reloading component companies came out with a loading manual using the 35,000 piezo number as maximum - but the next version of their manual reverted to CUP, and those loads were hotter. So it would seem 35,000 PSI may not actually be equivalent to 45,000 CUP in the .357.

[230RN]

Give yourself a "Like" from me on that, HankB.  I even got mixed up between CUPS and PSIs at one point.

Once again, try reading some of the information on CUP/LUP testing to understand how, and why, it was industry standard for decades before it was replaced.

CUP/LUP measurements worked perfectly well to determine chamber pressure. The methodology showed perfectly useful fidelity when compared against itself.

The fact that it's an obsolete measurement technology today doesn't invalidate the results that it provided.

That's the way I'm thinking.  It comes down to a direct "measurement" of pressure by how much force is required to duplicate the distortion of a pure annealed copper or lead slug, versus a "measurement" based on a number of assumptions including barrel material, heat treatent and tensile strength, barrel diameter, bore diameter, calibration of the electronic strain sensors, and a host of other variables.

I'm trending toward a vote for the former: direct measurement of a direct effect of pressure on a known material, which is completely independent of barrel materials and the other variables I cited.

A hole in a test barrel, any test barrel, a piston of any given diameter (area), a slug of known material, a compression tester, and a set of dial calipers. :)

Sometimes old fartery is superior to new fangelry.

Terry, 230RN

[K Frame]

"In Hatcher's Textbook of Pistols and Revolvers, he describes a pressure gauge using copper crusher cylinders, but refers to the crusher's pressure measurements in units of PSI."

Yep. That wasn't uncommon, even though it was known to be... suspicious and not a great metric.

And no, the term Copper Units of Pressure was in use before pizo or strain gauge measuring was developed.

The problem with all of this is that there was a LOT, and I mean a lot, of ambiguity inside the industry about how these tests were conducted. Government labs that their own procedures, and the various industry giants like Remington-UMC and Olin Winchester had their procedures.

You can set up your pressure test gun at any of several points on the case -- at the neck, just below the shoulder, mid body, near the case head. One company, I believe it was Peters, even experimented with case set back, essentially using the case head as the piston.

All gave different numbers. But even though those numbers were different, they were valid for that manufacturer.