New Pressure Testing Eqpt coming soon

25 August 2001, 02:04

Hot Core

Hey 1894, Over the years, I've found that most things which promise a quick and easy solution to a complex issue, are only "quick and easy" in the removal of money from "my" billfold.

The Ohler M43 is an outstanding tool for getting an idea of what Pressure a rifle is experiencing, but it is also not something I'd spend my money on. Reason being, it is getting "Secondary Pressure Information".

For me, the best place to get the Pressure information is directly from the weakest link - the actual Case by measuring Casehead and Pressure Ring Expansion.

A basic Physics course should be enough to convince a person that a "Recoil Operated Pressure Detector" just won't be accurate. In fact, anyone can even prove it just won't work before they ever spend money on the thing. Take an older experienced shooter and a less experienced shooter to the Range with you sometime. Set a target out only 25yds or so and have them shoot Off-Hand. While standing beside the experienced shooter, watch the path the tip of the muzzle makes as the rifle is shot. Then do the same thing with the less experienced shooter. Same can be done with two people of the same skill level, but having different body sizes (mass).

Physics just won't let a Recoil operated Pressure Indicator work. It could give you a "relative indication" of how hard one Load recoils when compared to a different Load by the same person, but my shoulder does that already.

Spend $35 for a set of 0.0001" capable Micrometers and you can get first-hand Pressure information directly from the Case on every rifle, revolver and pistol you have.

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Good hunting and clean 1-shot kills, Hot Core

25 August 2001, 02:07

Zero Drift

Saeed I agree. Knowing a little about Physics, I find it difficult to measure chamber pressures based upon acceleration alone. There are other dynamics involved in addition to momentum. Consider the size of the individual. Depending upon the mass behind the stock, momentum is transferred in dynamic ways that are not repeatable from shot-to-shot nor gun-to-gun.. The Oehler Model 43 system uses a strain gage attached to your barrel - that makes scientific sense. The Oehler is the next best thing to a true chamber test rig and costs under $800.

Sorry Hot Core, you beat me to the punch....

[This message has been edited by Zero Drift (edited 08-24-2001).]

25 August 2001, 02:43

Hot Core

Hey ZD, Lots of things to be "Sorry for", but AGREEING with me is not one of them. ;^)

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Good hunting and clean 1-shot kills, Hot Core

25 August 2001, 04:38

<Ken Howell>

Dr Kolbe's pressure-reading rig is his modern (?) version of the late Homer Powley's P-Max device. I have several P-Maxes (and the original dies for making the crushers used in 'em) among Homer's effects but haven't tried one yet.

Homer was this country's most accomplished and knowledgeable ballistician (interior and exterior, theoretical and practical). Dr Kolbe is a physicist, formerly from the University of California and the Lawrence Livermore Laboratory. Anyone who knows nothing whatever about the ballistics principles and relationships basic to the operation of Dr Kolbe's device would be wise to withhold disparaging "appraisal" of its efficacy.

I see no posts here from anyone whose judgment or expertise re interior ballistics I would consider more reliable than Homer Powley's or Dr Geoffrey Kolbe's. Like the rest of you here, I don't understand the principles of this specific device, and I don't remember which specific gun and load data go into its calculations. I have Homer's explanation here ï¿½ just haven't read it.

I have to wonder:
What "wisdom" enables people to be down on what they aren't up on?

[This message has been edited by Ken Howell (edited 08-24-2001).]

25 August 2001, 06:09

<Ken Howell>

Just looked-up and quickly skimmed Homer Powley's treatise, "Accelerometer for Recording Peak Pressure in a Gun" (pages 77ï¿½87 of the Frankford Arsenal Interim Report 28-MDC-A-76), which explains in detail the principles of operation of his P-Max device (although the Frankford Arsenal report does not refer to the Powley P-Max).

It all makes good physics sense, as anyone who knew Homer would take for granted that it would. It's all good, honest Newtonian physics.

Interestingly, a difference of 0.001 inch in the length of the squashed crusher represents a difference of about 300 lb/sq in., so ï¿½ as Homer writes ï¿½ "this method can be very reasonably precise."

Also "since the crushers are DYNAMICALLY calibrated, the results are closer to actuality than the usual copper crusher internal measurements based on static crusher calibration."

Use of the device and its equations requires using the area of the cross-section of the bore, the recoil force, the mass of the gun, and the mass of the device's inertia piston.

"If we know the caliber and weight of the gun and can find the peak rearward acceleration [which the device provides ï¿½ KEH] a simple reverse calculation will give the peak pressure developed."
...

"The peak pressure is now easily found by multiplying the peak acceleration by the mass of the gun and dividing by the bore area."
...

"It is to be emphasized that the recoil of the test gun is not to be impeded in any way; the recoil must be free when testing."

25 August 2001, 06:21

Vibe

Ken? What do you think the difference would be between a direct reading device and this device in a situation where the bore was partially obstructed (say with fouling build up in the throat), now load the round with a faster powder. Real pressures will be out the roof, while this device (even "properly" calibrated" could conceivably be showing "reasonable" pressures. Sorry Ken, there is just too much opportunity for other factors to "Indicate" a safe condition while none may exist. I would MUCH rather err on the side of caution.

25 August 2001, 06:45

<Ken Howell>

WADR, Vibe, your assumptions indicate that you're judging something that you clearly don't know all the details about. Obviously, there's a LOT more in the eleven pages of the FA report than I've excerpted in my post. I'm sure that you'd judge differently if you'd read the entire explanation.

A simple and all too common human frailty has reared its nasty head again ï¿½ "What I see here doesn't make sense, so this has to be a nutty notion (and what I see here is all there is)." There's a lot more that you haven't seen, obviously. I'm sure that once you'd studied the entire explanation, it'd make sense to you. But I'm not about to post the entire eleven pages!

25 August 2001, 07:20

Nitroman

While the piezo-electric method may be referred to as a "secondary pressure information" measuring tool, it has to deal with only one variable, the barrel.
I can tell you as a scientist, part of the difficulty in developing a good repeatable experiment is in dealing with variables.
With Oehlers device, attached to the barrel, variables are minimized. Once a set of readings are taken, I assume (having never owned one) operational deviations are blanked, therefore what you get on the machine is a relatively accurate picture of what is going on in the rifle. The transducer is integral to the barrel and the stretch characteristics of particular alloys of steel are known. If dealing with an unknown alloy, specific known test loads that develop known pressures with known pressure curves can be used to calibrate the machine

Measureing cases deals with many more variables: brass thickness, hardness, chamber variations will all come into play. Measurement error in the tool will be affected by temperature, skill of the operator (can you measure a case EXACTLY the same way each time) wear and tear, eye strain through the day. So on and so on.

This does not minimize the necessity of doing careful examination of your cases of course, I just do not think other means of pressure testing should be discarded before an accurate appraisal of their accuracy is known.

25 August 2001, 07:54

HenryC470

Is there any such thing as a copper crusher type device that wouldn't require drilling into the case or chamber? Let's say you could put a very tiny piece of material outside the case, but inside the chamber. Let's say for the sake of argument, against the outside of the case neck, which you've annealed prior to measurement and made a 0.002" dent in. Could you put a 0.002" annealed copper ball in that dent, fire the round, and measure the thickness or outside diameter of the squished ball and end up with an accurate pressure reading? I picked the case neck, because I figure that part of the case stays put under pressure and because you can anneal that part to constant hardness, unlike the case head. Back there, behind the case head, You'd have to worry about how forcefully the head would come back against the resistance of case stretch unless you had zero headspace. Also, controlling brass hardness back there is harder to do.

Just a thought. Wouldn't be surprised if some of you have tried something along these lines.

25 August 2001, 09:37

DB Bill

Anyone who thinks they can get reliable estimates of pressure by using a micrometer to measure changes in brass dimensions are kidding themselves..differences in brass, how many times reloaded, chamber dimensions, reloading die dimensions plus a whole slew of variables introduced by ambient conditions, the skill (or lack thereof) of the person doing the measuring, the calibration of the tool, and on and on and on.....you can get an "idea" maybe but it's about as reliable as old Aunt Tizzy's rhemeutism for predicting weather....maybe. And like those who believe I'm full of beans, I'm like you I'll never change my mind!

25 August 2001, 11:33

denton

What I don't understand is why any of these systems cost so much. A bonded strain gauge was $5 20 years ago, so figure $10 now, and the associated electronics are trivial. $3 for a precision voltage source, maybe $8 for an instrumentation amplifier, another $3 for a peak detector, plus $12 for a digital panel meter, and $8 for a decent case. There is no reason the whole thing can't be profitably sold for $129.

25 August 2001, 13:26

<Sandy>

I'm sure that I am about to show how dumb I am, but what does the symbol in front of the 500 stand for and what does it translate to in US $. Thanks.
Sandy

quote:
Originally posted by 1894:
I was talking to Border Barrels http://www.border-barrels.com about my 6x47 project and pressures and heard that they have a new pressure testing rig that will be released next year.
The unit attaches to the scope by a 1",26mm or 30mm ring. You input weight of rifle, weight of bullet, bore size and weight of charge and blast away - hey presto pressure reading for your load in this rifle (it measures recoil)
Will retail for approx ï¿½500
[This message has been edited by 1894 (edited 08-24-2001).]

25 August 2001, 16:08

<PaulS>

Sandy,
It is British pounds and used to be about twice the American dollar - It is less today but I don't know how much less.

Where is this description of the new device? I have a little physics background and would be interested in finding out how one translates inetial forces into peak pressure. You can get average pressures from such computations but chamber pressure has to be measured in specific maximum impulse to have any real meaning.

PaulS

25 August 2001, 16:24

denton

PaulS...

I gave that a little thought, and I think it is fairly straightforward.

You start with the fact that the center of mass of the gun-powder-bullet system stays in one place as the bullet travels down the barrel. Recoil starts as soon as the bullet starts to move. Since the bullet and burning powder are moving forward, the rifle moves backward. From the basic F=ma equation, you can easily derive the acceleration of the bullet vs. time if you know the mass of the rifle, the mass of the bullet, the mass of the powder and the recoil (acceleration) vs. time. The accelerometer gives you acceleration vs. time, and all the other numbers are constants. If you have the acceleration of the bullet vs. time, you know the force acting on it, vs. time because force is just pressure x area. The area is the surface area of the bullet. That's fixed. So the accelerometer can give you the last piece you need to complete the puzzle and get pressure.

The thing that I'm still chewing on is the issue another poster brought up: How does a human shoulder modify the acceleration? I don't know. You might have to hang the rifle from strings to eliminate that, or it might not be a problem. Or you could hold the rifle about an inch from your shoulder.... no, that's been tried.

25 August 2001, 18:00

<PaulS>

Denton,
let me play the "devil's advocate" here for just a minute.

for the sake of this arguement the parameters are:
two identical barrels
identical bullets
same temp, pressure, humidity and consistant ignition.

the #1 load uses a powder that burns twice as fast at the same pressure and temperature as #2. We want the same pressure so we have to use less of the faster burning powder. The #1 load produces 50000 psi at time mark 2 and the pressure drops off to 12000 psi at time mark 4.
The #2 load produces 50000 psi at time mark 4 and falls to 12000 psi at time mark 8.
Load #1 will only accellerate it's bullet to a velocity of x while load #2 accellerates it's bullet to a velocity of x(Pa2/Pa1).

The impulse of load #1 is lower than the impulse of load #2 by the ratio of the difference in average pressure. Even though the peak pressures are the same the impulses are not and neither are the velocities. If the velocities are not the same then neither are the recoil inertial forces. Any device relying on recoil is relying on the average pressure and not the specific impulse of the time-pressure curves.
Now that means if I load enough Bullseye powder in a 3006 to drive a 180 grain bullet to 2700 FPS in order to match the velocity of a normal charge of H4350 the peak pressure of the bullseye load would be enough to blow the gun apart. If the muzzle velocity is the same then it follows that the recoil will also be the same but I doubt that anyone would put 40 to 50 grains of Bullseye into a 3006 case and fire it.

This is the flaw in the idea of using recoil to determine chamber pressure.

PaulS

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stay safe and live long!

25 August 2001, 18:06

denton

Afterthought: Friction between the barrel and the bullet acts against the force of the burning powder. At 50,000 psi the force of the burning powder on the back of a .308 bullet is 3725 pounds. Bullet-barrel friction is about 150 pounds, but is hard to accurately measure. That's a 4%ish term that adds into the measurement error, unless you can somehow account for it.

That isn't a problem with the strain gauge method. I generally trust well done strain gauge systems to within a couple of percent total error.

It would be really interesting to try both systems at the same time, to see what conclusions you could really draw about accuracy.

25 August 2001, 18:21

denton

PaulS...

One key idea is that, contrary to what many say, recoil does not begin when the bullet leaves the muzzle. Recoil begins when the bullet starts to move in the barrel. Just as the bullet leaves the muzzle, its center of mass is at the muzzle, and the center of mass of the powder gasses is at the midpoint of the barrel. The rifle has to move backward as the bullet and gasses move forward, in order to keep the center of mass of the whole system in the same place.

Since all the numbers in the equations are constants, except recoil, if you know recoil vs. time, you also know pressure vs. time.

The end muzzle velocity is the integral of acceleration over time. The end muzzle velocity doesn't tell you anything about when, where, or how much the peak pressure was. But a graph of acceleration (recoil) vs. time has the same shape as the graph of pressure vs. time. All you have to do is combine all the constant numbers into one big constant, and multiply by that, and you've converted the recoil graph to a pressure graph. Look for the peak, wherever it occurs, measure the height, and you have peak pressure.

Because of barrel friction error, and the possible effects of of a human shoulder on the motion, I think I favor the strain gauge method. The barrel is a spring, pressure stretches the spring, stress is proportional to strain, and it's a nice, well behaved system to measure.

26 August 2001, 06:12

waitaminit

Hello to all of you - I am a newcomer to this board, and I hope I do not start here with a BS comment.

I'd like to refer to HenryC470's idea (copper disk in neck area).
I think that results will be more exact the closer you are to the load - any device between the source of expansion and the metering spot diffuses the results by additional factors.

I've seen a photo of a cartridge somewhere which had a dent close to the web in which they placed a little copper disk. Unfortunately, I do not remember what they reported about the results. As this method is no more applied, it obviously was not a hit.

I imagine a cannelure in the neck between bottom of bullet and shoulder, into which you place a copper wire of a given length.
After firing, the length is measured again, and the extension gives you a pressure reading.

Technologically, this may be a step backward, but you need not buy all these expensive electronic gadgets.

26 August 2001, 12:44

denton

I think waitaminit's copper wire idea is interesting. It would certainly be a cheap solution, and it might turn out to be pretty accurate. Just a little .001" wire that you could run the length of the casing....

The thing that bugs me is that a strain gauge system shouldn't cost that much. It would be hard to get $50 worth of electronic parts into the design, including the case and digital readout. Maybe I'll try tacking one together this winter.

26 August 2001, 17:38

Vibe

quote:
Originally posted by Ken Howell:
WADR, Vibe, your assumptions indicate that you're judging something that you clearly don't know all the details about. Obviously, there's a LOT more in the eleven pages of the FA report than I've excerpted in my post. I'm sure that you'd judge differently if you'd read the entire explanation.

A simple and all too common human frailty has reared its nasty head again ï¿½ "What I see here doesn't make sense, so this has to be a nutty notion (and what I see here is all there is)." There's a lot more that you haven't seen, obviously. I'm sure that once you'd studied the entire explanation, it'd make sense to you. But I'm not about to post the entire eleven pages!

Ken. I'll concede that you know a LOT more than I do about reloading. But I've been a mechanical engineer for 15 years and did machine design for 12 years prior to that. I know what accelerometers can do, and what they can't. There are WAY too many things happening during the firing of a projectile that involve internaly restrained stresses. These are the forces that are acting upon, and being reacted to by, portions of the firearm completely internal to the mechanism, and thus do NOT have a measurable EXTERNAL component. The device in question, that mounts to the scope, will NOT measure pressure. Will it give you a COMPARATIVE measurement of the differences in recoil of various loads? Absolutely, and probably a very good one. But to imply that it will MEASURE chamber pressure in any believable way is simply wrong. It can only estimate what that pressure was IF you can input all the other factors accurately. This is quite different that actually measuring that pressure. And I for one am not about to bet what could quite likely be my life on the difference. When I need to know chamber pressure, I will measure it with either a strain gage or piezeo device, when I want to know recoil I'll use an accelerometer.
If you 11 page report is factual, it will tell you the same thing. If it's not, I have no interest in it.

26 August 2001, 18:00

denton

Vibe...

I think you may have made an error in your assumptions. It is not true that internal forces cannot generate an external force component. What is true is that the center of mass of the rifle-bullet-poweder system must remain fixed unless acted on by an outside force. Since the bullet and the propellant gasses move down the barrel, the rifle MUST move backward to keep the center of mass of the total system fixed. Forward movement of two internal components absolutely requires the rifle to move backward as the bullet accelerates through the barrel. In an ideal system, the graph of acceleration vs. time is just a multiplicative constant away from a graph of pressure vs. time, while the bullet is still in the barrel. (NOTE: Recoil DOES NOT just happen as the bullet exits. The physics get very different then, but a lot of recoil happens while the bullet is in the barrel.)

The problem is that the system is non-ideal. I haven't yet figured out the effect of having the butt of the rifle against a shoulder as it is fired, and the bullet-barrel friction term is not negligible.

All in all, there is a good mathematical case for the notion that you can indeed derive pressure as a function of time from acceleration (recoil) as a function of time. All that said, the basic strain gauge system is a lot cleaner, and very cheap to implement. I don't see a good case for using an accelerometer... never liked 'em anyway. Too fussy to work with... heat sensitive.

27 August 2001, 09:15

waitaminit

Denton, I confess I did not know that recoil starts with the bullet still in the barrel, and confuses me when I see bechrest techniques where they touch hardly more than the trigger - with extreme accuracy results.

I don't know much about Kolbe's stain gauge as well, but I think that not touching the gun for a strain test would be one of the imperative operational rules (benchrest situation again) to achieve correct and repeatable results.
I imagine a mini accelerometer to read the movement in the three directions x,y,z and a mini computer to calculate the results, and a mini screen to show it to you. This sounds not like a scope-sized device and will not meet my mini budget anyway.

Thank you for giving my copper wire idea a positive consideration. It hopped to my mind about ten minutes before I entered your site and hopped off already: I'm sure that others have had that idea before me, and it proofed to be so weak, that it was not even good enough for a photo. Not good enough, because most of the energy will go into flattening the wire, and little into streching it lengthwise due to the longitudinal friction.
In addition, calibrating of the extensions found would be a chore.

I agree with you: a real pressure gauge is the best way to go.

[This message has been edited by waitaminit (edited 08-31-2001).]

27 August 2001, 01:22

<Ken Howell>

I have neither interest nor incentive to try to persuade anyone to buy or use Dr Kolbe's pressure device. Nor do I question anyone's familiarity with other applications of accelerometers, etc. I can not respect negative opinions based on total ignorance and hasty assumptions made AFTER the "conclusion" that the device can't do what both Powley and Kolbe say that it does (and explain how it does it).

If you start with the "conclusion" that any device is nonsensical, without any familiarity with it or any understanding of its logic, then all that your "thinking" allows you to consider are "reasons" that it can't be logical or sound.

What sucks is denunciatory "judgment" based on absolutely no knowledge of the principles or procedures that make such a device logical to those who DO know its rationale.

Also less than worthy of great respect are comments that question the device on the basis of the effects of the shooter's shoulder on the reliability of the device, after the clearly worded caveat in Powley's explanation has just been posted.

So I quote ï¿½ again ï¿½ this line I've already posted, from Powley's explanation:
"It is to be emphasized that the recoil of the test gun is not to be impeded in any way; the recoil must be free when testing."

Who here remembers the classic line about trying to appear wise and merely appearing foolish? I'm having a senior moment and can't remember how it goes or where it comes from. ï¿½o)

27 August 2001, 04:21

Vibe

Ok. Let me simplify my position to the completely absurd degree. What is the recoil on the system (shoulder included or not) if the round is fired and the bullet DOES NOT MOVE. Does recoil THEN have any bearing upon pressure? This is an example of COMPLETE internal counteraction of forces. This being the absurd degree case let us assume that the rifle does NOT self destruct. Would one think that the chamber pressure was indeed close to Zero? Would you think that this were a SAFE assumption? This being a Border Barrels device being discussed wouldn't you take testing information from their own website as somewhat factual. Information that indicates that a 0.3% reduction in bore diameter resulted in a 5% rise in pressure. Does this mean that you MUST be able to input you bore diameter to the 0.0001" for the accelerometer to work? Or from the same testing, the bearing surface length of longer bullets also plays a part in pressure values due to nonlinear drag, particularly in tight bore barrels. This being a classic case of INTERNALLY counteracted forces which result in rises in chamber pressures yet a reduction in bullet/rifle acceleration.
The addition of a restraining body mass (IE a shoulder) is not nearly so crucial, since that can be accounted for by adjusting for the rifle weight. In an IDEAL, frictionless, perfectly repeatable world, one could "get away" with assumptions that make the derivation of chamber pressure from the resultant recoil possible. I haven't visited that world since I left college. Beginning Physics used that world to simplify the problems enough for newbies to understand, that doesn't mean that the results of those calculations had any basis for analogy in THIS world however.
And Ken...I do not believe that I have made any attempt to insult YOU or anyone else in this thread. Nor have I tried to imply that the devise would not measure recoil like it is intended to do. My only position is that there are too many opportunity for internaly counteracted forces to affect the system for ANY recoil force measuring device to in any way be considerd as able to MEASURE chamber pressure. If I am mistaken in this, then educate me. Because apparently Engineering school and all my years of experience have been wrong.

I'm glad to see you back from the hospital.

I'm sorry to see that you came back thinking every post that disagreed with your opinion was a flame.

27 August 2001, 07:48

denton

I think Vibe hit on one of the fundamental problems of an accelerometer based system, and that is that bullet-barrel friction is hard to predict. A different finish on the bore, .0001" difference in bore diameter, and different "stiffness" in the material the bullet is made from would make each system a little different. The question is whether the differences are enough to worry about. Barrel friction is a few percent of the forward force of the propellant gasses. There may or may not be enough measurement variation introduced to worry about... only testing would tell.

Of course, if you stop the bullet from accelerating, there will be no external motion. But, then, you've made a model that doesn't match the real world. The real world is a piston in a cylinder, being accelerated by compressed gas.

It's been fun to chew on this, and to learn about it with you guys. That's what makes this fun. My take on the situation is that the product can work, I'm a little worried about how much accuracy can be achieved and some of the testing constraints, and I think there are a couple of more direct routes that are known to give an accurate result with no more cost or effort than this one. So it probably works, but I don't see any great advantage in it.

27 August 2001, 10:27

<Ken Howell>

" ... then educate me...."
All right, give me a little time, and I'll try to work up and post a brief version of Homer's explanation. I'm sure that you'll find more solidity there than you've so far assumed that there was ï¿½ including some that I suspect that you haven't considered.

"I'm glad to see you back from the hospital."
Thanks! The one delightful development in all this has been the massive evidence that I have a lot more good, true friends than I ever would've guessed.

" ... you came back thinking every post that disagreed with your opinion was a flame."
You couldn't be more wrong if you tried to be as wrong as you could ï¿½ and succeeded. I haven't posted in this thread anything that I'd call a flame, and I haven't read anything in this thread that I'd put that word to. What did you use to measure my attitude? A strain gauge? What did you mount it on? ï¿½o)

27 August 2001, 11:33

<Ken Howell>

excerpts from Powley:

"In a gun the bullet is propelled down the bore by variable pressure provided by the powder gas. Gas comes from burning powder. The pressure during burning of the powder continues to rise until burning is almost completed. Just before completion of burning the pressure reaches a peak value. The value of this peak pressure has considerable significance and importance. The problem is, how to measure it.

"The peak pressure is reached in an ordinary rifle approximately 600ï¿½800 microseconds after burning begins and remains at the peak value only a few microseconds.... Meanwhile the gun has recoiled something like one sixteenth of an inch during the time required for the pressure to reach its peak.

"Since pressure is force (in pounds) divided by area (in square inches) you can easily calculate the force at various values of peak pressure in various calibers of guns. Simply multiply the peak pressure by the area of the bore. Following are some typical values for the peak force, in pounds.

[Table shows peak forces for several calibers at pressures of 40,000, 50,000, and 60,000 lb/sq in. In a caliber-.30 rifle, for example, these peak forces are 2,904, 3,630, and 4,356 pounds, respectively.]

"These peak forces are the momentary maximum values producing forward movement of the mass of the bullet and about one half the mass of the powder gases. From Newton's third law, these peak forces are the SAME which are applied to the gun and cause movement of the gun to the rear (recoil).

"Since the pressure and thereby the force is variable the recoil is variable while it is happening. Variable force is variable acceleration acquired by the gun. We are interested now in the peak acceleration of the gun.

"All you have to do is divide the peak force on the gun by the mass of the gun to find the peak acceleration of the gun. F = m ï¿½ a = (w/g) ï¿½ a
a = Fg/w

"Following are some typical values for the peak acceleration of a 10-lb gun:

[Table shows peak acceleration values for several calibers at pressures of 40,000, 50,000, and 60,000 lb/sq in. In a ten-pound caliber-.30 rifle, for example, these values are 9,340, 11,690, and 14,000 ft/sec/sec, respectively.]

"It is now easy to see that if we know the caliber and weight of the gun and can find the peak rearward acceleration a simple reverse calculation will give the peak pressure developed.
...

" ... let us use a lead crusher cylinder held against the rear of one of the recoiling surfaces of the gun. Behind the crusher we hold an 'inertia piston,' ... a known weight which is in contact with the crusher but quite free to move.

"When the gun is fired, the crusher will be compressed, or shortened, since it is between the recoiling gun and the piston. Heavier pistons will cause more crushing than light ones, so a suitable weight can be selected. With a fixed weight of piston and gun the crusher shortening will record the peak force exerted on it.

"From Newton's second law the recoil acceleration of the gun is equal to the acceleration causing movement of the piston and crusher.

"With carefully made lead crushers and suitable calibration methods we can know how much force it takes to shorten the lead crusher by various amounts. So by measuring a shortened crusher we can know the peak force applied when the gun was fired. Now we can divide this force by the mass of the piston to find the peak acceleration, which is the same for the gun as for the piston....

"The peak pressure is now easily found by multiplying the peak acceleration by the mass of the gun and dividing by the bore area. P = w ï¿½ a/32.16

"Following is a sample table showing the crushing of 3/8 ï¿½ 3/16 lead cylinders:

[Table shows dynamic forces (pounds) to shorten a 0.375-inch-long lead crusher to a number of test lengths from 0.370 inch (50 pounds) to 0.280 inch (423 pounds).]
...

"A difference of 0.001 inch in measurement makes a difference of about 300 lb/sq in. so this method can be very reasonably precise.
...

"The accelerometer will properly record the maximum pressure which results in motion only but not the additional 6% or 7% further required for overcoming friction and heat losses.
...

"It is to be emphasized that the recoil of the test gun is not to be impeded in any way; the recoil must be free when testing."

[Other material goes into further detail, including how to calibrate the system.]

Maybe I also need to post the obvious ï¿½ Homer tested and checked this system thoroughly and carefully before he wrote any of the above. He was easily able to compare the results from his P-Max device with a good number of data from pressure guns fitted with transducers ï¿½ peak pressures as accurately known, in other words, as it was possible to measure and record them. This is just one more reason that I can not credit the nay-saying of laymen who (a) don't know the rationale behind the system and (b) have no data comparing this system with any other. Forgive me if I'm wrong, but I seriously doubt that anyone who's posting here can ï¿½ after the briefest, most cursory consideration of only a few words about the system ï¿½ immediately and intelligently conclude logically that it's nonsensical.

27 August 2001, 17:23

Vibe

Makes perfect sense..IF..If you are interested in peak NET chamebr pressure. But since piezeo, strain gage, and copper crusher syetems are designed to measure peak GROSS chamber pressure, these two systems do not in fact measure the same thing at all.
I understand all too well the theory at work here, and have understood it from the first posting.

Powley is correct in his assertations, so long as we limit the discussion to that PORTION of the pressure which is resulting in motion, that portion of the pressure whose force is applied to overcoming the friction between bullet and bore is not accounted for in his system. Yet this portion of the force is oftentimes not insignificant, particulrly with tight bore barrles and long bearing surface projectiles. Moly adds a whole nother variable into the mix as well. One would assume that the reduction of friction between barrel and bullet would bring the system more closely in line with one which could be measured in this way, however the reduction in friction also tends to reduce the peak pressure (at least temporarily). But in some calibers (most notably those considered "overbore") this also leads to a buildup of hard carbon in front of the throat, bringing about a delayed (by several firings) increase in peak chamber pressure. While all of this is going on there are changes in the recoil signature of the rifle, but they are not always indicative of the changes in peak gross chamber pressure. Indead at sometimes the changes will be seen to be in opposite directions.

It is interesting to note that what Powley had set up was a Balistic Pendulum with a peak impulse recorder built in. It is and was an ingenious device to record some quite intersting data. But this data does not in any way indicate exactly what the gross pressure was during any given firing. Unless one is willing to assume that gross pressure and net pressure are ALWAYS proportional to each other in the same ratio, or at least are tied together by a known function. I do not think that that has been shown, in fact that assumption has been shown to be flawed outside of the application of some VERY controlled conditions, involving a very small sample of VERY meticulously maintained firearms, in a very small sample of calibers.

I've never claimed that the device would not measure NET pressure (which produces recoil), only that the claims that it could measure GROSS pressure (which can and will rupture a barrel) were inaccurate, and that these two pressure values could and do vary by quite a bit.

The reason I find the use of a balistic pendulum so interesting, is that I have used a very similar setup in leu of a chronograph on a few occaisions.

27 August 2001, 19:31

<Ken Howell>

Then who wrote (above):
"The device in question, that mounts to the scope, will NOT measure pressure. Will it give you a COMPARATIVE measurement of the differences in recoil of various loads? Absolutely, and probably a very good one. But to imply that it will MEASURE chamber pressure in any believable way is simply wrong. It can only estimate what that pressure was IF you can input all the other factors accurately. This is quite different tha[n] actually measuring that pressure."

I won't bother you with this again, my friend.

28 August 2001, 03:06

Vibe

I wrote it. And to my way of thinking it does not conflict with what I have been saying all along. It will estimate Net pressure, if that is what you wish to refer to recoil as, but it will not in anyway measure gross pressure, which is what we are really interested in.
I think we are both clear on it now as well.

28 August 2001, 03:47

Hot Core

Good thread which has "once again" shown it is best to post with a bit of education and experience on a subject rather than having to paste something in from a book that you simply don't understand.

I've been afield hunting, so I didn't intentionally abandon the Thread.

Looks like Howl's first post is his typical improperly rationalized, personal attack on me: "I have to wonder:What "wisdom" enables people to be down on what they aren't up on?". But then he later says he has "seen nothing that resembles flames". As a response, I'll just say, No Comment from me, or he might have a REAL stroke!

Hey Vibe, You did right well in your posts UNTIL you got to: "Ken. I'll concede that you know a LOT more than I do about reloading." If in fact that is a true statement(which I suspect you have been mislead into thinking is true), I'd STRONGLY encourage you to use Factory Ammo.

Other than that, just excellent posts Vibe. I enjoyed reading your thoughts. Thanks for the well thought out input.

Hey DB Bill, Funniest post I've seen in a long time. Almost seems as if you are trying to justify 100 times the expense of the Micrometers on a piece of equipment that will give you "Secondary Information" and only 10% of the Mic's "useful" information.

If you have access to a M43 Owner's Manual, read the "Calibration Instructions" and sit back and think about the fact an "unknown" is used to do the Calibration.

Have no idea at all if you are "full of beans", but I'll take your word for it and try to stay "up-wind". ;^)

Enjoyed your post very much. I do admit there are a few "tricks" to getting accurate, usable information with the Mics. But, I'll not try and change your mind. Good luck to you.

Hey Denton, Last year a poster had provided a link to a device such as you described and I think it was right at $130. I just tried to look up that link and it no longer exists. I enjoyed your posts on this subject as well.

Hey PaulS, Excellent point about different Powders will change the entire result. That also kills this Rube Goldburg idea. Once I got to the point in my thinking through the Physics(not long at all) that it was simply a "billfold emptier", I didn't look for a second way to shoot it down. But, I do agree with you and enjoyed your post as well.

Hopefully this Thread will save a few folks some money.

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Good hunting and clean 1-shot kills, Hot Core

[This message has been edited by Hot Core (edited 08-27-2001).]