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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.
Well, I'm going to respectfully disagree again. There are two forces at play to get either the primer or bullet to start moving - (1) you have to overcome the static friction holding it in place, and (2) you have to overcome the inertial mass. The primer starting to move first is because the force holding it in place (friction) is lower AND the inertial mass is lower. It's not because the pressure is higher down near the primer. Where's Denton when you need him?
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I did NOT say "the pressure is higher down near the primer" I said the powder starts to burn there, so the hot gas expands through the flash-hole quicker than through the neck at the other end of the case. Plus, the primer is obviously easier to push backwards, the reason fired .30-30 cases often show "high primers."
Just because the peak pressure is evenly distributed throughout the inside of the case doesn't mean pressure doesn't affect everything in the same way during the initial powder burn.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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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. Apparently, the Steve to which you are referring is not me. Still, I am sorry that i did not preface my post with a brief explanation. I was attempting to point out that it was hard at that time to keep up with the science. It was evolving quite quickly. Much like electronics and computers are changing now. For example, I was thinking that Hatcher's work (and the work of others) was done before test equipment or methods had been devised or refined that could accurately determine over pressures within a case. Some things that people of the time believed have since been disproven. Using primer flattening or expanded bases might have seemed like a logical conclusion then, and few would have questioned it. Hatcher worked for the Ordnance Corp and his opinions were respected, and deservedly so. But science is always advancing, both the methodology and a better understanding of the chemistry. Explosives and the science that explained it were advancing almost daily at the beginning of the 20th century. I have said this before, but in a very short time, we went from primitive single shot mechanisms using BP and firing lead balls, to smokeless, repeating firearms firing jacketed bullets, tracers, etc. It would have been an interesting time in which to live. What you learned in university was quickly eclipsed by new science. Men like Hatcher worked for the Ordnance Dept for over 40 years and they saw a lot of changes. I think Hatcher and others can be forgiven if they didn't get everything right. Their work advanced our understanding of ballistics.
Safe Shooting! Steve Redgwell www.303british.com"Comparison is the thief of joy." - Teddy Roosevelt. Member - Outdoor Writers of Canada ![[Linked Image from i.imgur.com]](https://i.imgur.com/h0cT3TP.jpg)
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Steve,
That particular post was directed toward Steve692.
Would love to hear if you have evidence that Hatcher was wrong about primer appearance. None of the piezo-lab people I've talk to believe he was, for the reasons stated in my article: There are too many variables in primer cups, and other factors such as headspace, bolt-face, etc., for primer appearance to work even as a rough "estimate" of pressure.
Which is why I ran experiments on such traditional pressure signs with the help of one of those piezo labs over 15 years ago. One .270 load averaged around 69,000 PSI, with some of the individual shots over 70,000. Yet the primers appeared completely "normal," just as normal as the primers used in a .30-06 load that averaged 58,000 PSI.
In fact the only "pressure sign" that followed the piezo results was velocity.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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That's out of my area, John.
If I was attempting to determine that primer appearance was a reliable indicator, it would be necessary that absolutely everything was uniform, test to test. All the components of the test cartridges, the firing mechanism, etc. In other words, you would have to positively eliminate differences in propellant/primer composition, case construction, ambient temperature differences, use a single test mechanism, etc. That's an awfully tall order. Temperature and pressure would have to be constant, cartridge to cartridge. If you could manage that, then change one thing at a time to see how much of an influence that change had.
I don't see how that could be done with our present technology.
Safe Shooting! Steve Redgwell www.303british.com"Comparison is the thief of joy." - Teddy Roosevelt. Member - Outdoor Writers of Canada
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I did NOT say "the pressure is higher down near the primer" I said the powder starts to burn there, so the hot gas expands through the flash-hole quicker than through the neck at the other end of the case. Plus, the primer is obviously easier to push backwards, the reason fired .30-30 cases often show "high primers." True, you did not exactly say that. You implied that pressure is felt on the primer cup before the bullet base because the powder starts burning (resulting in expanding gas) closer to the primer. This implies that primer moves back first because it "feels" pressure buildup first (before the bullet feels it). I don't think that's the way things work . . .
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Steve,
I have provided several examples of how primers can give "false positives" of too-high pressure, and one example of a "false negative."
The exact sort of test you are describing occurs all the time in piezo labs, in very controlled conditions. If primer appearance (or even measurement) provided a valid measurement of pressure, why would professional labs go to all the trouble and expense of purchasing all that expensive equipment?
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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MuskegMan,
The primer doesn't "feel" anything. Instead, the flash-hole is just the first "escape channel" for hot powder gas, just as the lowest point in river levee is the first for high water.
Once again, the reason primers back out very early in the build-up of gas-pressure is because they're far lighter and less "tight" than the bullet. I already gave you a very graphic example with .30-30 ammo--but a less obvious example is MORE primer flattening in new brass in higher-pressure cartridges. That occurs because there's often a little more headspace with new brass, again because the primer backs out before the case starts to expand. If the same load is fired in once-fired, neck-sized brass, the primer won't be nearly as flat.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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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 Not a strain gauge system, but I was always intrigued by the York-Cantrell "system".
Not a real member - just an ordinary guy who appreciates being able to hang around and say something once in awhile.
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Not only a less than minimally educated person, but stupid and out of touch as well.
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John, they wouldn't. Labs follow a scientific method. Have you seen anything that indicates they have been successful following a particular metric?
Safe Shooting! Steve Redgwell www.303british.com"Comparison is the thief of joy." - Teddy Roosevelt. Member - Outdoor Writers of Canada
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Steve,
Their methodology is all laid out in the SAAMI testing protocols. You can see them on the SAAMI site--but at least one lab I've worked with considerably runs their own experiments now and then.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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I know, John. They keep testing to verify and advance the science. Because I never worked in development, I have relied on the testing that the powder/explosives companies employ and trust their data.
I only worry about pressure when I'm testing new powders. Then, I depend on my chrono. Some might call me naive, but pressure and velocity are closely tied together. I never worry about primers because I was told years ago, despite due diligence, things fail from time to time. They are the exception, not the rule. Try not to worry about the "what ifs".
Safe Shooting! Steve Redgwell www.303british.com"Comparison is the thief of joy." - Teddy Roosevelt. Member - Outdoor Writers of Canada
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Might also mention that have had extensive conversations about copper-crusher testing with pressure labs, because even as major labs have converted to piezo, they have plenty of copper-crusher barrels on hand. which some still use for cartridges that aren't all that popular, because it doesn't make sense to build/buy a piezo barrel. The main comment from the head guy at one lab was that copper-crusher barrels provide good overall pressure data (though not as precise from shot-to-shot, pressure-curve) as piezo barrels.
But the biggie is takes a lot more time to do the prep and post-work work for a CUP test. It takes far longer than most shooters would imagine just for a valid for a piezo test, including far more expensive equipment, but not nearly as much time as with CUP equipment, mostly because of all the measurements involved.
Yes, Hatcher did use copper-crusher equipment, which was state-of-the-art in his day. But it was perfectly capable of providing results to come up with conclusions about pressure and primer appearance.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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I have never had a blown primer or stuck case. Thanks to Muledeers information that I gleaned years ago about useing a chronograph and velocity as a pressure indicator I may very well make it many more years to come. Thanks Muledeer!!! No doubt I've been fortunate and probably some luck involved but good information goes a long way toward helping
Trystan
Good bullets properly placed always work, but not everyone knows what good bullets are, or can reliably place them in the field
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Trystan,
Good to hear the chronograph has provided reliable results!
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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Question - Does anyone know if Hodgdon uses the copper-crusher or the piezo-electronic method for measuring pressure for the 257 Roberts? Their site shows CUP but is that really how they test?
There is no accounting for taste.
Experience is a great thing as long as one survives it.
Generally, there ain't a lot that separates the two however,
Barely making it is a whole lot more satisfying than barely not making it.
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It means copper-crusher is how they tested the data listed.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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Yes, I understand that is copper- crusher. They don't test with the Piezo method for the 257 Roberts? I had a discussion with a gentleman about pressure in the 257 Roberts a while back and he didn't believe Hodgdon's data. He was running it off of a computer simulator program and it was reporting over pressure. I figured they were testing in Piezo and converting it to CUP, but I suppose I was wrong.
There is no accounting for taste.
Experience is a great thing as long as one survives it.
Generally, there ain't a lot that separates the two however,
Barely making it is a whole lot more satisfying than barely not making it.
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Don't know of ANY pressure lab that tests peizo and publishes in CUP. Why would they?
I know of at least two companies that still sometimes test less-popular cartridges with copper-crusher equipment, because there isn't enough demand for the round to justify the cost of a piezo barrel.
Am always astounded at how many handloaders believe computer-simulated data is somehow more accurate than pressure-tested data.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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I don't know of any reason for it either. I figured they(Hodgdon) would have given up testing in CUP by now and perhaps converted back to be consistent, and was surprised by new data being published in CUP.
A number of years back I wrote a simple application that converted from CUP to PSI. It was loosely based on relational data and was less reliable in the extremes of pressure data. The thought was they may have a more accurate conversion with all the data they possess.
I didn't make my sense to me why someone would believe a simulation over tested data, but there it was. He was obdurate.
There is no accounting for taste.
Experience is a great thing as long as one survives it.
Generally, there ain't a lot that separates the two however,
Barely making it is a whole lot more satisfying than barely not making it.
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