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I thought some might find this interesting. It is meant to explain the truth as to why pressure estimation by "pressure ring" or "case head" expansion is basically bogus. Quote: Flame away children. All I ask is that you use physics and NOT opinion to flame me. ASS_CLOWN | ||
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BTT. No flames, I am mightily impressed! ASS_CLOWN | |||
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AC... The physics is a little wrong, but the reasoning and the conclusion are correct. If nothing else limited the effectiveness of PRE and CHE, the inability of a micrometer to resolve small enough differences would be enough. The expansion of the barrel is not the key issue, though. It does stretch for a few milliseconds, but gun designs keep this stretch within the elastic limit of the steel. It snaps right back into shape when the pressue is released. The issue is how much the brass case goes into "yield". It would be wonderful if you could get good peak pressure estimations from changes in brass dimensions. I wish you could. But, as you correctly posted, you cannot. If you could, the same pressure would reliably produce the same amount of change in a case. It does not. The real limiting factor is probably inconsistencies in the physical properties of brass, which are an even greater barrier than the resolution problem. I ran an experiment to find out how good PRE and CHE are, and the results were not pretty. The problem is that people apply the method, and their gun does not blow up, so they think the method works. Fact is, you're not probably going to blow up a modern rifle with any load that isn't just ridiculously overboard. So you do the numbers, your gun doesn't blow up, and you figure the system works: Voila! The illusion of success. In my tests, I fired several cartridges with pressures from 60-65,000 PSI. Case head expansion ranged from 0 to .0004". You have to do a lot of averaging to overcome a system with that much random error in it. The CUP and piezo systems are fair, but less than wonderful. They give you about two significant digits. PRE is better than CHE, and it has just enough resolution to tell the difference between a 44,000 PSI plinker load, and a 73,000 PSI barrel buster. | |||
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Which all leads me to the conclusion that use of a chronometer and micrometer to develop accurate loads and keep pressures within the range that allows me to reload my brass more than several times may be wrong in theory, but correct in practice. | |||
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HiWall... I still end up a little conflicted over the whole issue, because I'm not sure how much confidence I have in SAAMI specs. You have to remember that for several decades, people thought that the copper crusher method gave results in PSI, and actions were designed accordingly. That turned out to be wrong. Anyway, I don't think your micrometer is going to be much help to you. It's kind of like Dumbo's magic feather... lets you confidently do something, with very little real substance behind it. After my experiment, I'm convinced that you can load a 70,000 PSI barrel buster, and very easily fail to detect it with either the case head or the pressure ring method. On the other hand, the chrony can be a big help. If you're getting the book velocity, chances are very good that you're at max pressure. The chrony is a real eye opener. My own interests drive me toward instrumenting with a strain gage, but I know that's not everybody's cup of tea. | |||
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Denton, My original post is directed toward pressure determination through hoop strain measurements of a rifle barrel shank. In that regard, I assure you the physics are correct. I left out all the derivations to work through the triaxial stress / strain system to arrive at the relatively simple equation posted. Believe me when I say hoop stress, radial stress, and longitudinal stress are all considered in that hoop strain equation I posted. My point toward PRE and CHE was that the measurement systems employed in the application of these "methods" are GROSSLY inadequate to predict anything other than massive pressure differences. A more accurate method would be like the one "Clark" uses here at AR, namely load the case till the primer blows and back off 10%! It is relatively easy to calculate the pressure required to blow a primer.  It is also relatively well understood how much pressure each powder will produce per unit of mass.I would also like to say that CHE is WRONG, as it requires the head to plastically deform. If this is indeed occurring one has simply destroyed a case!! In a mauser case head this requires ~ 70,000 psi! The case head IS a solid disc of brass with only the flash and primer holes in it. The weakest point being within the primer bore which is supported by the extractor groove. PRE occurs forward (toward the neck) of the case head at the forward terminal of the transition radius between the case head bulkhead and the case walls. Here I agree with you 100% regarding the difficulty associated with getting REPEATABLE brass deflections since each case will have a slightly different physical dimension as well as slight variation in mechanical properties, coupled all with head space variations case to case which will place this terminal position in variable positions relative to the taper of the chamber, etc, etc. It would be a statistians wet dream I suppose to try and comprehend the degree of significance each of the variables poses though. Regarding the SAAMI or CIP pressure measurements, they are not very accurate even at the 1000 psi increment, due to the lack of resolution they allow themselves i.e. thick barrel sections. Strain gages, in order to be accurate, need to measure relatively significant magnitudes of strain. ASS_CLOWN | |||
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AC... We're probably in violent agreement on most points. It's absolutely true that a micrometer does not have enough resolution to reliably measure the temporary increase in diameter of a barrel. Where I think we're not connecting is that the barrel expansion is pretty much independent of the brass expansion. The expansion of the barrel is well within the useful range of a strain gage, and an excellent indicator of chamber pressure. From published data, and from my own tests, I estimate the standard deviation of the random error (sigmaE) of the available pressure measurement systems to be CHE method: 7,500 PSI PRE method: 6,800 PSI copper crusher method: 1827 PSI published piezo PSI method: 1366 PSI PressureTrace strain gage: 667 PSI Of course, smaller is better. Commercail piezo data would be a lot better if they would control barrel temperature as carefully as they control ammo temperature. Anyway, long and short of it is that a careful home experimenter can produce pressure readings at least as repeatable as the commercial labs do. | |||
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I won't argue that micrometers are not accurate enough to measure accurately true absolute pressure because of brass inconsistancy etc. etc. etc...... However I believe that it does miss the point that they can show signifigant enough changes between different loads, i.e if all of a sudden your expansion suddenly increases signifigantly it's time to back a good bit off! I believe that the old Waters idea of using a factory load as a control and then measuring to make sure you don't exceed it is still a valid sign of pressure changes as long as you realize that it is only one of the factors you are observing.........DJ | |||
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Hmmm, a murky world this Internal Ballistics stuff. I do not have the benefit of pressure testing equipment, nor the experience/education Denton has on the subject, but I have reached a few of my own conclusions. One is that the least reliable method of evaluating pressure in your loads is any of the techniques recommended by the "scribes". Been there, done all that. IMO, when you get to the point where you DO have difficulty with bolt lift, blown/cratered/flattened primers, expanded case heads, etc. ad nauseum...you've gone way too far. WAY too far.  The suggestion made by DJ...I like it and it scares me. I cannot duplicate factory components. Unless the factory ammo is used solely as a velocity reference to see what SAAMI spec's will do in your rifle... And I've had some factory ammo in my life that was WAY too hot. Heaven forbid that some chump try to match velocities with some "Heavy-Mag" or somesuch! I do like to use .22 RF ammo(match) for a "calibration" on my Chrony. I also agree with Denton that If you've matched book velocity with your loads and it's verified on a chronograph, you're okay. Sometimes I can't though. And there is my K-Hornet that shoots 200-300 fps faster than Hodgden suggests with less than a Max load. A lot less than max.  Like 8%.  Some time back I thought Exterior ballistics was complex, and it is, but nothing like Internal and Terminal. Life was simpler then.  Denton, thanks for your posts here. Did you contribute to VHA Mag awhile back on this subject or something close to it? AC, thanks for bringing this up. I realize you were trying to get to the point, and I agree with your end result, but maybe you should have included the physics. I don't believe that barrel expansion is relevant to estimating pressure by lay persons trying to evaluate handloads, if for no other reason than most of us can't do it. | |||
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Dan... Yes, it is a fun topic. I find batting it back and forth to be very helpful, whether I agree with all the posts or not. I did have a recent article in VH about which measurement systems you can/can't trust, plus another one coming out in a couple of weeks on temperature, primers, and powder. In October, they are planning to publish the complete workup on the PRE and CHE methods. The only methods that I trust now are these: 1. Shoot factory ammo 2. Reload, and follow the book 3. Slap a strain gage on the barrel, and verify it with a couple of different commercial loads. There is one big factor that SAAMI does not control, and that is barrel temperature. In my tests, it was about 3X as important as ammo temperature. If you want really consistent results, you need to strap a thermocouple to the barrel, just forward of the receiver, and do all your testing at constant temperature. To a reasonable approximation, in a 30-06 size case, 1 grain = 2,000 PSI and 50 fps, and 3 degress F in the barrel = 200 PSI and 5 fps. 30 F barrel temperature is about equal to a grain of powder. You are absolutely right that by the time you see "pressure signs" you're way the heck and gone over the limit, and need to back down about 4-5 grains in an '06 size case. | |||
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Denton, I tend to agree that we are in "violent agreement". Your statistical analysis: Quote: Matches closely to my calculations and what I have measured myself. Hench my comments with regard to the published pressure data. Your pressure trace system numbers seem incredibly low. What cartridge(s) and barrels dimensions did you use when gathering that data? . Also, what calibration was performed, ie did you instrument a barrel and use hydraulic pressure to generate a calibration curve for your instrumentation system. You do realize, I hope, that strain gages / barrels don't play exactly to the numbers (for many of the same reasons you cannot measure brass directly) so a calibration curve IS required to get the accuracy you seem to be posting. ASS_CLOWN | |||
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Denton, you're probably aware of the article a few years back in VHA about a fellow named Hall and his 'War Wagon'? I mention this in regard to your comments about barrel temp. He was shooting prairie dogs with water jacketed barrels hooked up to pumped chilled water that was regulated by thermostat. I vaguely recall that they maintained 57*F on the barrels like this despite high rates of fire. Evidently they had some very good accuracy as well, since the author claimed 1st round hits to 1200 yards. I don't know how this would play regarding strain sensors but assume it is doable. As if it weren't complicated enough. I've long considered this a more significant factor than heat induced distortion in barrels. I've seen M-60 machine guns continue to fire after trigger release due to heat transfer(cook-off?), so I know that a warm/hot barrel can pretty much make hay out of one's efforts to keep ammo temp. stable. Brass is a marvelous thermal conductor. A last comment/question: John Barsness recently wrote that fluted barrels skewed pressure readings with a Oehler 43 when he was doing some work on a couple of different guns at a well know gunsmith's shop. When questioned about this on a web site I don't think he knew how to explain it, and I mean nothing derogatory by that. I surely don't either. Have you any experience with this and/or an explanation? He called it "Fluted Gobbledygook" or something like that. | |||
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Quote: Hey AC, Kind of a bold statement that is obviously totally full of beans. Good old Case Head Expansion(CHE) and Pressure Ring Expansion(PRE) have work extremely well for over 100 years to help "knowledgeable" Reloaders (like HiWall, djpaintles and me) achieve SAFE MAX Loads. So, there is no real need for me to heap praise on Pressure Detection Methods that have easily stood the test of time. ... But then we get to the Strain Gauge Systems(SGS) and they have all kinds of insurmountable problems with them when "attempted to be used" by anyone outside a recognized Ballistics Lab. So many problems with them that it is almost difficult to know where to start. 1. Cost. I understand the "cheap" ones run about $200 from some of the posts here and I know of others that have spent $2000-$3000 to get an Oehler to where they "think" they can get some useful data from it.Of course you can get totally usable 0.0001" capable Micrometers for under $20 which will provide better and more relevant information. In a Ballistics Lab, the cost is not an issue. 2. Where to "Stick Them". Ah yes, the old what to do with the Strain Gauge. You can always glue it on the outside of the chamber. I actually prefer the SGS users do this to indicate to the rest of us sharing a Range that they obviously have ZERO idea of what they are doing, which allows us to stay far away from them.Or the Strain Gauge can be hidden in the stock which requires "gouging out" the stock and/or bedding to make clearance for it. Then the wires hanging out look real cute. And of course "accuracy" is definitely compromised. But the folks "typically" interested in home SGS are more interested in "reading an inaccurate PSI value" than they are in "rifle accuracy", so this doesn't matter to them at all. In a Ballistics Lab, Strain Gauge placement is not an issue. 3. Barrel thickness at the Chamber. Since the SGS users constantly "deride" the accuracy capability of "0.0001" capable Micrometers", it is obvious they would never attempt to use one to establish a basis for knowing how thick the barrel is where they intend to "stick" the Strain Gauge. So that leaves them with the "less precise" 0.001" capable Calipers and a common yard stick. (As you will soon discover, they are simply "guessing" at the barrel thickness, so the yard stick is just as useful.)Most barrels have an ogee curve located over the chamber area. (This means it is a double curve for the SGS folks.) And of course the chamber has a Tapering Curved surface. Using "ANY" measurement device that does not have a "Ball Tip" to measure either the inside or outside of the Barrel Reference Point means the measurements are not precise - just guesses. Even if the home SGS user had access to a $250,000 CMM where they work, few of the CMMs have the correct Ball length to be able to reach through a Receiver and get inside the Chamber in order to measure the Internal Reference Point. So, they can use a yard stick and "guess" measurements as "accurate" as they claim to get. Of course none of this "guessing" is necessary with CHE and PRE. In a Ballistics Lab, doing this measurement is not an issue. 4. Calibration Let me say this clearly, the home SGS user is "completely unable to Calibrate the SGS". You could shoot factory ammo until the barrel falls off and unless you have access to Certified SAAMI Calibration Ammo, you are only "guessing" about Calibration.The Micrometer user can self-Calibrate. In a Ballistics Lab, obtaining the SAAMI Calibration Ammo is not an issue. ... Those four insurmountable problems face any home SGS user. They can do the measurements however they want and Calibrate however they want, but the end result when read from the Strain Gauge is at best "a poor guess" at the Pressure. In comparison, CHE and PRE never claim to tell the person a Pressure. However, both CHE and PRE allow the user to "Compare" their Reloads with the CHE and PRE allowed by the Ammunition Manufacturers with their Millions of Dollars of Test Equipment. Quite a bargain. And you get to use a Pressure Detection Method with over 100 years of excellent performance for less than $20. ... No desire to argue any of my totally, 100%correct, points. ... Quote: Well AC, Let me say this about that - You sure caught him there. You are pointing out one of the Major Problems with SGS owners - most of the home SGS users claim accuracy that is totally impossible. This misleads people who really don't have the proper background into thinking, "Why, he must be right. Look how precise that data is!". For those of you who really don't understand what AC caught, go back and look at Denton's "data". I seem to remember him "claiming" the SGS is accurate to 667psi. To be even close to "reality" it would have been much better for him to have said "My best guess is the SGS is capable of maybe 1Kpsi resolution." This means the 1s, 10s and 100s digits are insiginificant, which would be far closer to reality than 667psi. But in order to get even 1Kpsi resolution, it must be done in a Ballistics Lab. Once folks realize all this home SGS data is based on "guessed at measurements" with "absolutely ZERO Calibration", then they can decide for themselves how accurate and usable it is. ... Best of luck to all you folks - especially the home SGS users! | |||
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AC... It's interesting to meet someone who has actually done some of the same measurements! In my case, I got the piezo data from the test done on p91 of the Lyman book. They show 10 PSI readings from 10 identical cartridges. If you simply take the standard deviation of the data, you have an upper limit on the standard deviation of the measurement system error. For the PressureTrace system, I was able to do it a little more elegantly. If you fire matched pairs of cartridges, and keep track of the differences between them, there is a little math thing you do to deduce the standard deviation of the measurement error. PRE and CHE royally flunked this test for self-consistency. Since it is just a measure of self-consistency, you don't have to make any assumptions about the mean. For this calculation, you don't even care if you're calibrated. The whole thing depends on differences, and any bias (lack of calibration) just washes out in the math. The Lyman piezo data have a very definite trend in them, which is what got me to thinking about barrel temperature in the first place. If you whip a regression onto the data, and look at the residuals, you have de-trended data. The net result of that little adventure is that if SAAMI were controlling barrel temperature, the piezo data would be as good as the PressureTrace data. If memory serves correctly, it got down around 450 PSI, but with this small a sample, you can't really tell which is better. I also have a Fabrique strain gage system, and I tested it, too. It's about as good as the CUP system, but it isn't as good as the PressureTrace. The circuit board is not as carefully done, and I think it has a tendency to pick up stray RF out of the air. So all strain gage systems are not created equal. Burt did an exceptional job on the PressureTrace. Dan... Water cooled?? Now that's someone that is serious about controlling temperature. Unfortunately, if the accuracy of a rifle depends on having a load that is tuned to within half a grain, that is about what you have to do to retain the accuracy. Yeah, I'd be very concerned about getting accurate readings off a fluted barrel. The symmetry of a normal barrel really helps you. I'd be thinking about FEM if it came time to instrument something like that. The accuracy of the strain gage system depends on how good the hoop strain equation is, how well you know the physical properties of steel, and how accurately you can measure a couple of dimensions. I would expect the hoop strain equation to start having trouble with a fluted barrel. Fluting may be counterproductive, anyway... not that I've done any experiments of my own, but Harold Vaughn did FEM on barrel mechanics. His conclusion directly contradicted the "conventional wisdom". His modeling says that a barrel should be as heavy as possible, but that excessive rigidity is bad. My "take" on it is that it raises the Q of the barrel, and tends to increase the amplitude of the barrel vibrations. | |||
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Denton, I totally agree with what you have to say about barrel temperature. But in my opinion there is a caveat to it that has not been address thus far in this thread. The more significant issue with barrel temperature, again in my opinion and based upon experience with measuring pressures, is the dwell time the loaded cartridges sets in the heated barrel. It takes time for the heat transfer to occur, so the longer the cartridge sets the the barrel the more dramatic the thermal effects. Hot core, Not saying the PRE won't point out high pressure loads, just trying to say it's accuracy is VERY GROSS. More so than a properly set-up strain gage system, that is fact. DigitalDan, An M60 machine gun cooking off rounds. I have NEVER heard of such a thing. Could you please explain this phenomena to me in greater detail. FWIW I have seen M16, AKM, and FAL cook off rounds before. ASS_CLOWN | |||
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In a nutshell, doesn't the variation and lot-to-lot inconsistencies in the properties of brass pretty much negate PRE and CHE as truly viable, repeatable methods? If not, please, educate me. RSY | |||
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AC... I think you're absolutely right about the time a cartridge spends in a hot chamber... good point. After a few minutes the powder is going to be pretty close to the chamber temperature, and the starting temperature of the powder won't matter at all. | |||
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Denton, I hadn't really thought of "Q" (as in electronic resonant circuits) as applicable to rifle barrels, but it's a good one. I've read a statement that is explained by the "Q" concept, or degree of resonance damping: "Free floated barrels tend to shoot very well, but are picky about loads. Barrels with a contact pad at the foreend tip may not group as tightly with the best loads, but shoot a much wider variety of loads reasonably well." That suggests to me that the conatact patch is damping the vibrations. Damping (lower Q) not only reduces the peak amplitude and shortens the decay time of resonant oscillations, it spreads out the frequency range of the system's resonant response. Best accuracy should occur when the bullet exits near a reversal of the barrel's vibrational motion, as the free end is always going to be a "loop" or antinode. With a high-Q barrel, the resonance peaks will be sharp and the bullet's exit time must very precisely match that period of the barrel's vibration to hit a steady point in the muzzle's movement. With the resonance peak spread out by vibrational damping, it's less likely that you'd hit an exactly stationary point in the barrel's movement, but if it's slightly off of that it's far more likely you'd hit a point at which it's only moving a little than if you slightly missed the reversal point of the sharply resonant, high-Q barrel. | |||
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Quote: Hey AC, Well, by gosh I do agree totally with "the first part" and hasten to add it shows a smooth Expansion characteristic as you increase the Powder "when" using a Powder that works well in a specific cartridge. Like IMR-4064 or Varget in a 308Win. I've just not found the accuracy to be "VERY GROSS". But, I'm in no way trying to make it do something it was never intended to do. It has never been stated by anyone that CHE and PRE can be read to indicate "PSI", though many folks have tried to make it do so. It just dosen't work that way. I can agree CHE/PRE accuracy would be VERY GROSS if done incorrectly. ... Quote: And I would even TOTALLY AGREE with the above statement if it was slightly changed to read, "A SGS can give excellent Pressure Data when "properly set-up" inside a Ballistics Lab that has a CMM to obtain the proper dimensional measurement accuracy for the Chamber wall and the recognition ability to obtain SAAMI Calibration Ammunition." Otherwise, folks would get the TOTALLY MISTAKEN impression that a SGS is somewhat useful for them when Reloading at home. | |||
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1) Cartridge brass as a test instrument: a) With the same pressure, thin barrels expand elastically more than thick barrels. The load required for radial elastic limit for the brass then can be affected by the chamber wall thickness. http://yarchive.net/gun/ammo/cartridge_expansion.html b) Extra hard 26000 cartridge brass could vary from 79 to 87 ksi yield strength. I have looking for for evidence of this. With 7.62x25mm 110 gr. the primer falls out with 14.2 gr. AA#9 in Starline brass, and 15.3 gr. with S&B brass. Both pieces of brass have the same internal volume and weight. I finally found one, 7.7% extra powder. AND the extractor groove on Starline is .3370" the S&B is .3315", with the small primer pocket at .172", the S&B is 3.4% thinner. If I could add these together [in my dreams! the magnitude does increase] the S&B brass must be 11.1% harder. http://www.technicalmaterials.com/metal_prop/brass.html c) I now believe that measuring a change of .001" in extractor groove diameter is a more precisely reproducible test than the threshold of the primer falling out. This becomes obvious when with a radical overload, the primer fails to fall out, but the extractor groove expands appropriately. 2) Q a) I think I one hits the barrel barrel with a hammer, in or out of a stock, one can hear the self resonance of the barrel, with the sustain and amplitude a function of the Q. http://encyclopedia.thefreedictionary.com/Q%20factor Sorry for the following digression from pressure... b) But if one hits the barrel with the palm of the hand, while gripping the wrist of the stock with the other hand, [the rise time of the attack being much slower] one can hear the tuning fork formed by; the fore stock, butt stock, and barrel. I try to get as much Q as I can, as it is an indicator of stiffness of my glass bed job. -- A society that teaches evolution as fact will breed a generation of atheists that will destroy the society. It is Darwinian. | |||
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Clark, Once the brass is plastically deformed, which it must become if you are to measure a difference in size before and after firing, how do you directly compare that expansion in the extraction groove to pressure? I agree that you can approximate, but not necessarily very accurately, the pressure ring expansion to pressure. The ony way I know of to do this is to ASSUME that the brass, after firing and plastically deforming, as created a "cast" of the fired rifle chamber. There are a great many assumption in there, each one raising red flags of inaccuracy and uncertainty. Dan, I guess I am a little confused as to how the ammo gets so hot in a M60 that it will cook-off. As I said, I have seen M16s, AKM, and a FAL cook-off ammo. With those rilfes the solution is simple, you simply must drop the magazine. They cook off because they are loading cartridges into the chamber, since they are all closed bolt rifles. The M60 is open bolted so the ammo is a few inches from the hot barrel, thus my confusion. So if you could provide a bit more detailed description of the events leading up to M60 cook-off I would appreciate it. What I have seen a machine gun do is BLOW a very hot barrel up! That is most uncool. ASS_CLOWN | |||
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Denton, The easiest method is mass damping as you alluded to. The other approach, in it's simplest form, is to increase the dynamic stiffness of the barrel, this basically means one increases the static stiffness at a greater rate than the mass. There are ways to to the latter, albeit none that I am familiar with are all that appealling to the eye. ASS_CLOWN | |||
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Quote: Hey AC, Well by golly, we are in agreement again. Had some time this afternoon and pulled out a very old(1988) Mitutoyo Measuring Instruments catalog. Took it outside, got a good comfortable chair and began looking through it. I wanted to see if they possibly offered "anything" at a reasonable price that would allow the "Home SGS" user any chance at all of being able to measure the Chamber wall thickness. I thought they might have some l-o-n-g reach, Ball tipped Micrometers that "might" allow reaching into the chamber with the Barrel still attached to the Receiver. Didn't find anything suitable. Even looked at the Black Granite Surface Plates and Digimatic Height Machines. A "small load" 24"x36" was only $805 back then. The book even listed the old mechanical(non-computerized) CMMs. Now that really brought back some "bad" memories. While siting outside day dreaming about the impossibility of accurately measuring at Home, your point about axial positioning went across my mind as: 1. A slightly "Eccentric Chamber". 2. A slightly "Out-of-Round Chamber". 3. A slightly "Distorted Chamber" due to Receiver Tension. The "Home SGS" just gets in deeper and deeper the more you think about it. ... Quote: Yes, that is one of the "easy" ways to tell if the person who has one actually understands the "limitations" after all the set-up guessing. I actually fault the manufacturers of the devices here, as much as the people who buy them that just don't understand what they are getting. The manufacturer could truncate the Data Output at the 1K level. Or they could have a short paragraph explaining the info "isn't as significant" as the LEDs will lead(mis-lead is more accurate) you to believe. Of course, then that would let the cat-outta-the-bag. So, yes indeed I totally agree with your "Primary Point". In fact, I really doubt "ANY" of the "Home SGS users" have the "Set-Up" done with enough precision to achieve even that level of accuracy. | |||
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1) In guns that are stronger than the brass, the limit is the brass, and loading to a pressure would introduce and error. 2) In guns not considered stronger than the brass a) Loading to a pressure becomes more attractive. b) Overloading guns until the chamber splits, backing off a safety margin, and using that load for similar guns is something that should be done [and I have], but expensive and dangerous for the individual. | |||
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Ah, Hot Core. You continue to strain at gnats and swallow camels. Surely, there is a photo of you next to the word "cavil" in the dictionary. One of the great beauties of physics is that it applies everywhere, not just in the lab, and that anyone who properly applies the principles will be rewarded with the same results that happen anywhere else in the universe. She works the same for me, for DD, or AC, or anyone else who picks her up and puts her to work. The point is, that, in the hands of a competent user, a strain gage system WILL resolve differences as small as 1,000 PSI or so, which is within 2% or less, which is quite good. This has been verified by experiment. Neither the PRE nor the CHE method even remotely approaches that level. By actual experiment and calculation, PRE, the better of the two, has a 95% chance of resolving a 29,000 PSI difference. This has also been verified by experiment. You were even asked to participate, and declined. But you do have endless comments about those who actually did the work. Ken Waters, who you said you'd never contradict, was extremely clear in saying that PRE didn't even qualify as a measurement system. And he was right. Do an experiment, get some data, or actually read the sources you quote. Until you have some actual data, I have no time for you. | |||
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Clark, you have some good points, as usual. Since the objective is to keep the gasses in the gun, until after the bullet exits, it's good to know how much it takes to break the gun. But you are braver than I am. (And richer??) One question never answered to my satisfaction is, do SAAMI pressure specs actually mean anything? My current theory is that they do, and that they represent an economical operating point, where the probability of catastrophic failure is low over many rounds, and brass life is acceptable. My main worry about exceeding SAAMI specs is that there is a fairly well proved theory that says that if you draw a curve with log strain on the X axis, and log N on the Y axis (where N is the number of exposures), you get a straight line downward to the right. Bottom line: The number of events required to produce a 50% probability of catastrophic failure in N trials declines exponentially as the applied stress increases. A relatively small increase in MV produces a relatively large decrease in firearm life. I look bad enough as it is. I'd look even worse with a rifle bolt where my nose once was. I have to be careful. | |||
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Quote: Ah denton, You say that as if you think "I care". Just stating my thoughts. If they expose your foolishness and make you look like you really don't have a clue as to what is going on with a "Home SGS", then that is just the way the old cookie crumbles. If you go back and look at "your posts" you are the one misleading the unknowing by claiming "significant" levels of accuracy in the 1s, 10s and 100s with your "Home SGS". Pitiful! Of course, I realize you really don't understand why that is so. But, I do admit your posts on, "How Great a SGS is!", are always good for a bunch of laughs. | |||
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Quote: And you base this statement on what data? Oh, I forgot. Based on your opinion, which we are to accept over all data to the contrary. Let's recount the history of this discussion. You put yourself forward as an expert on PRE and CHE, quoting Ken Waters to all of us. Then someone pointed out that what you were preaching wasn't the Ken Waters method at all. Ooops. Then you had 20 years of experience with strain gages, and knew they wouldn't work as advertised. A little pop quiz exposed that as a fraud, and it was evident that you know next to nothing about them. Then I proposed a test to see if PRE and CHE work, and invited you to take the measurements, with the results posted here for all to see. I said that we didn't have enough good data to decide whether they work, and ought to check it out. You turned tail and ran like a scalded 'fraidy cat. Then you turned to name calling. Now the results are out, and they don't support your point of view, so all you have is sneers for the people who rolled up their sleeves and went to ask Mother Nature what works. You have no data, and no basis for complaining about those who do. Protect your ignorance well. Once it is lost, it can never be fully restored. Or at least lay off the beers for a few hours before you post. | |||
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So how do you successfully damp a barrel? If we can figure that one out, we can be famous, as long as the solution does not involve something like 200 pounds of concrete at the muzzle. How about this? It's basically a delrin pressure block that can be adjusted in position along the barrel axis by means of a threaded rod/adjustment screw available through a small hole in the fore end. There are no other cosmetic changes, since it uses the front sling swivel stud for most of its mechanics. | |||
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Interesting idea. Even though it talks about an "attenuating block", I think what they are doing is tuning the resonant frequency of the barrel, much as a BOSS does. Or are they sliding a pressure point in and out, to damp barrel oscillation? | |||
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Sliding, statically adjustable pressure point. | |||
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Hmmmm.... Might also give some useful function to the cleaning rod hole that is left when you sporterize a Mauser stock. | |||
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I think a couple of things are getting lost in the argument. First measuring expansion is a rough estimate. I agree. The other point is that ass clown is proving that the method also works. In fact he shows in the .375 .0008 exp. is 57000-63000 roughly psi. this is at the ragged edge of max. in this case. The non-suprise is that for the last 25 years I've decided that .0007 is the edge and .0003-.0005 expansion is quite safe. According to his calculations, he is proving that my old method can be used to predict pressure, not to a close degree, but that you won't hurt yourself if you never have over .0005 expansion. I would love to have a more absolute way to measure pressure, but see too many variables in the strain gauges as sold by Oehler, etc. I was very interested in one of these until I read the details and they require calibrating with factory ammo. To me that would be like calibrating your speedometer by setting it the same as your neighbor's car because he says it's right on. Remember we're not capable of computing absolute pressure, just prevent a blowup, and find a place to stop. | |||
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JS.. I really wish that it worked that well. It would be very convenient if it did. I went into this project with no position taken, for or against PRE and CHE. I simply proposed a test to find out. The result of the test was very clear. Variation in brass properties is great enough that you can't get the same answer twice, given the same input conditions, except by luck. It even swamps out micrometer resolution, which is on the ragged edge of being a problem to begin with. Both the PRE and CHE models contain more random error than they do information. You can defeat random error by averaging, so the question becomes whether or not the number of cartridges you have to average to get reasonable precision is practical. It is not. You have to have a standard to compare to, and to get the random error in the PRE method down to the point that it is comparable with the old CUP method, you have to average 140 known cartridges. Well, OK, 138, and I rounded. I have no emotional attachment to any particular measurement system. I'm just looking for the easy way to get useful data as cheaply as possible. You can get there with PRE and CHE, but they are wildly impractical, compared to other methods. They are also dangerous. When I said that PRE could only resolve a 29,000 PSI difference, it wasn't a number plucked out of the air. It was a number arrived at by careful experimentation. So, if people want to believe that it works, that's their prerogative. The data contradict that notion, quite firmly, but nobody has to believe the data. Now, how do you calibrate a strain gage system? The basic theory is the same as calibrating a balance scale. There are only standard masses (quantity of matter). There are no standard weights (force of gravity on a mass), unless you specify where in the universe you are when you make your measurement. A standard mass will vary 1% in wieght as you move it around on the Earth's surface. But, no worries, we use standard masses to calibrate a scale, and multiply by the acceleration of gravity, and simply mark our scale in grains or pounds. We convert by formula from mass to weight. You do the same with a strain gage. The expansion of the chamber is a very linear function of pressure (Hooke's Law). Pressure can be expressed as a simple function of the properties of steel, and two dimensions. The properties of steel and the two required dimensions can easily be obtained to three signficant figures, and related to pressure by formula, just as with a balance scale. The factory ammo is just a safety check to make sure that you did everything correctly (no bubbles under the gage), and that you entered the numbers correctly. The larger question, which Clark reminds us of, is does all of this mean anything? Experiment and analysis tell us that we are getting reasonably good pressure measurements with a strain gage, but I still don't have a good answer to Clark's question. Worse, I still haven't figured out a way to get the answer. | |||
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AC, somehow I never had my nose under the hood to see what was going on when this happened. It is possible I suppose that something else was at play. I'm not lettered in thermodynamics or mechanical engineering and had other priorities at the moment. The reports sounded odd, were on occasion repetitious, and I saw more than one door gunner break a belt to stop it. I am guilty of assuming that they cooked off, but in any case it was...annoying. I know without question that -60's get hotter than Hades after sustained fire. You think you'd want to stick your pinky anywhere near there after a long burst? I think the chamber/throat area is near incandescent myself. Oddly enough, I never saw a -16 do this. Saw a Ma-deuce blow one day and that wasn't funny. Helmut visors are a good thing. | |||
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Quote: I was surprised to learn how soon engineers figured out crushers gave low readings. An article by E. Harrison in the 8/1975 issue of the American Rifleman had details. The acceleration of cannon carriages was measured in the 1800s using a stylus on a tuning fork to mark a metal foil. From this, pressure could be estimated and it was obvious the crusher was low. With regards to engineers designing actions to the pressures indicated by crushers, this may have proven to be conservative, assuming modern proof standards were used back then. SAAMI wants a proof load of at least 130% of the working load, regardless of whether crushers or piezo is used. I would guess a similar % was used back then as well. Because the amount crushers tend to underestimate pressure increases with pressure, proofs can end up being more severe with crushers. Consider the .303 British which is spec'd at 45,000 CUP and 49,000 psi. From these, SAAMI then recommends 65,500 psi as a minimum proof load, but with crushers they want 60,000 CUP which is very likely a far higher pressure, on the order of 80,000 psi. | |||
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Quote: Based on your post from the first page of this thread which both AC and I already explained to everyone else is Full-of-Beans: Quote: ... Quote: I really don't remember it that way. I believe I was just returning what I got. But, it doesn't matter, cause I'm not proud of it. Though I do realize most all of your postings on "Home SGS" are foolish and based on a lack of understanding, I really don't consider that "name calling" for me to point out how inaccurate and worthless a "Home SGS" is. Simply stating my opinion. Quote: Perhaps it will help you save a bit of time and aggravation for me to explain that I'm in absolutely "no way" interested in any "advice" from you. I'll do my best not to respond directly to you in the future. But I'll retain the right to point out the inherent problems of a "Home SGS" for people that are considering wasting their money on one. Once folks realize all the "Home SGS" data is based on "Set-Up guesses", can't be "Calibrated" and a $20 Micrometer will serve them better, they can determine for themselves "Why?" anyone would try to PT Barnum them into buying one. .......... It is quite interesting to note that the longer this thread is around, the more we see people joining it who have excellent luck with CHE and PRE. And none of them dispute the "inherent problems" of the "Home SGS". Maybe the value of the "correct" message is getting out. .......... Ha, even another entire Thread of folks having excellent luck with CHE and PRE. http://www.accuratereloading.com/ubbthreads/showflat.php?Cat=&Number=691431&page=0&view=collapsed&sb=5&o=2&fpart=1#Post691765 | |||
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DigitalDan , I guess where I am having the difficulty is this. The ammo belt is behind the chamber and the bolt is open. So if you are cooking-off ammo in a M60 the ammo would go off in the belt! I sure as hell would not what to be anywhere near that thing if the ammo was going off in the belt/tray, let alone "break the belt". I think I would be seriously contemplating which would be worse 1.) jumping off that bird 2.) getting shot or hit by M60 belt shrapnel Too bad you never looked "under the hood". I would very much like to understand what was really going on in there. ASS_CLOWN | |||
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AC, I've been pondering that a bit since you questioned it. Upon reflection I'd have to say I don't have a clue. Probably there was a mechanical going on rather than what I said, unless there were hangfires involved. Bad ammo lots were not unkown then. The mechanical geometry of the -60 stands against a cook off as you say. During my first tour we had great diffficulty obtaining parts and eventually discarded the -60's in favor of the M16 for Scout Gunners due to extreme lack of reliability. As much as I saw them fired I did it only once at a range, never really looked at them very much. I "knew" how they functioned but never connected the dots. Thanks for the refesher course. And for the puzzle. I'll probably never figure that one out. What I witnessed, I witnessed, now I wonder why... I still wouldn't put my pinky near one of those things after the fact. They DO get hot.  Nothing I used to know is still true. | |||
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DigitalDan, I definitely would not put my finger anywhere near the breech of a "warmed up" machine gun! That is why you get that nifty asbestos glove to swap the barrels with!  The most interesting bit I can comment on with machines guns is that their accuracy doesn't seem to be too adversely effected by barrel temperature. Unlike all these off the shelf hunting rifles. Wonder why? It is too bad you cannot recall the exact nature of the defect, as I would have really liked to know what was up with that! Oh well, sh_t happens, seems to get worse the older we all get. ASS_CLOWN | |||
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