I'm way out of my league on this one but after doing a little research the 17.5 Ton proof converts to 38,526 PSI. That isn't a heck of a lot. What am I missing? Please educate me!! That is slightly more than HALF the pressure the 6.5 x 55 Swedish Mausers were "Proofed" at, i.e. 65,000. And yes I know that the 65,000 PSI is a conversion of the CUP actually employed.
I was thinking the same, myself, but what do I know? I'm just an American and those blokes speak English, a rather peculiar language.
Then I found this on another forum:
Excerpts:
This sort of confusion is what happens when you apply common sense and logic to British measurements.
I found this quote several times while searching for "British proof":
"Per the 1954 Rules of Proof, here are the equivalent service pressure values:
3 tons--8,938 psi "
Don't ask me how that works.
Anyway, 6 times 8,938 psi = 53,628 psi = 18 "tons" per square inch, about right for a 30-06.
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The problem appears to be that way back when they used a Lead Crusher Radial system to proof firearms. When they began changing over to a newer system they had to settle on a number to use in the conversion formula because they couldn't figure how the old system complared to the new one. I'll quote a guy who appears to know a lot about it.
http://kwk.us/pressures.html
"No accurate conversion between crusher and true pressure exists, but approximations can be made"
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Pre-metrication British chamber pressures are:
1. Based on the long ton of 2240 lbs,
2, Measured with an axial crusher gauge with element in a modified bolt head, not a radial gauge with hole in barrel.
These combine for lower numerical readings than US radial crusher gun CUP and lots lower than piezo psi.
It does not mean that the rifle has not been thoroughly tested and it is not unsafe with fresh ammo.
Your gun is fine.
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Gough Thomas correlated British and Yank pressure ratings and concluded that a long ton of load on a axial crusher was equivalent to about 2800 lbs dead load on a US type radial crusher gauge.
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The 'meat and potatoes':
Cartridge Pressure Standards
In the U.S., voluntary pressure standards for rifle cartridges are set by
SAAMI, a member of
ANSI. Most other countries in the world follow the standards of Europe's
CIP (
Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives or Permanent International Commission for the Proof of Small-arms, sometimes referred to as the International Proof Commission).
Conversion Formulas
No accurate conversion between crusher and true pressure exists, but
approximations can be made. In all the conversions here, pressures are in ksi. Expect errors of several ksi, or about 15%, with such formulas. Many factors determine how much the indicated pressure reading from a crusher misses the true pressure, and the error varies among cartridges and even among different loads for one cartridge. The following conversions might be accurate enough for many practical purposes.
In Denton Bramwell's article, he offers a formula he derived using a basic statistical analysis of SAAMI's ratings, covering only pressures between 28,000 and 54,000 CUP :
piezo = 1.52 * crusher - 18
He also demonstrates that within this pressure range, the CIP appears to have generally used a simple conversion between their crusher and piezo ratings, roughly equal to:
piezo = 1.21 * crusher - 2.8
CIP pressures are multiples of 50 bar (about 700 psi), probably rounded after the conversion. (Please note that CIP crusher readings should not be equated with SAAMI CUP crusher readings; see below.)
In the 09/1968 issue of
Handloader, Lloyd Brownell
presents test data (crusher, but not necessarily CUP) which suggests to me a linear conversion formula is not the best choice, and in my
Powley Computer I use:
piezo = crusher * ( 1 + ( crusher^2.2 )/30000 )
From 0 to about 60 ksi crusher, it fits both SAAMI's ratings and Brownell's data well, but it is low at the high end of Brownell's data. Brownell's data shows little to no error below 20 ksi, and a curve fit to only his data between 20 and 67 ksi crusher is:
piezo = crusher + ( (crusher - 20) ^ 2.3 ) / 210
Reference Ammunition
Under SAAMI specifications, reference ammunition is required only for the qualification of new pressure barrels. A new pressure barrel must demonstrate it generates nearly the same pressure and velocity as existing SAAMI spec. pressure barrels. Reference ammunition is as uniform as possible, and ideally all pressure barrels will show the same indicated pressure and velocity. If one barrel is found to be different,
something is off in either the barrel or its sensors.
Reference ammunition is
not used to calibrate pressure sensors. Piezo systems are calibrated hydraulically. Crushers are calibrated by the manufacturer. (The use of reference ammunition to try to correct crusher measurements is listed as "optional.") To quote SAAMI: "Reference Ammunition cannot guarantee the absolute accuracy of any test system."
CIP procedures permit the use of reference ammunition to correct pressure readings taken at any one lab. The reference ammo has been fired at several trusted labs, and the average of these readings is the reference value. Reference cartridge pressures measured at any other lab are compared to the reference value, and if the difference is less than 10%, the offset can be used as the correction in subsequent tests at that lab.
Proof Loads
For rifles, SAAMI recommends a proof load between 33 and 44 percent over the nominal rating, and the CIP today requires 25 percent over their rating (an older standard called for 30% over). While SAAMI requires only a single proof firing, the CIP wants two firings, except in long guns designed for low pressure cartridges (under 26 ksi), where only a single proof cartridge need be fired.
For handguns, the CIP uses 30 percent over, while SAAMI varies the proof load with the rated pressure. For cartridges rated over 20 ksi, SAAMI uses the same overloads as with rifles, but low pressure cartridges have a higher overload, with those rated under 15 ksi having a minimum of 44% over.
To conduct a proper proof, one would ideally need precise gauges to verify no stressed part has yielded (
ie., taken a permanent deformation) in the slightest. If no yielding occurs at the proof pressure, then the gun should have an adequate fatigue life at normal operating pressures. I've read that in practice, visual inspections are permitted on production guns, so I suspect properly instrumented proofs are only conducted on prototypes.
Interestingly, the same percentage overload is used with both piezo and crusher ratings at SAAMI. Above, it was noted there is evidence that crusher's underestimation of pressure grows ever worse as the true pressure rises. One curious side effect is that rifles proofed with crushers may well be proofed to a higher standard than those proofed with piezo.
Under the British base crusher standards described below, proof loads ran 30 to 45% above normal. To maximize breech thrust, proof cartridges were oiled before firing.
Kynamco in England still rates their production cartridges with this method.
Piezo systems have been available since the 1920s and today are the accepted standard, but other systems to produce a pressure trace were tried. Vieille in France, who developed the first military smokeless powder, also "invented a rotating recording crusher gauge with which pressure could be measured as it varies with the time." I have no details on Vieille's crusher, but I've read he was the first to detect pressure waves inside the chamber. The Russian researcher Serebriakov employed in 1923 "a conical crusher" to investigate burn rates in a calorimeter—again, no further details. In the U.K., J. J. Thomson began experimenting with piezo systems during WW-I. A system demonstrated to the Springfield Armory in 1921 had the chamber piston press against "a very stout stiff steel bar, which had a polished end forming a reflecting surface to act as a mirror." A light beam was reflected onto a revolving drum of film to record the pressure trace.
Piezo systems measure the displacement of electric charge in a crystal as it is compressed. For SAAMI tests, a piston in the side of the chamber is cut to conform to the case. This leaves the brass cartridge case between the gas and the sensor, but the sensor is calibrated by hydraulically pressurizing a test chamber with a case in it, and in this way the effect of the case is known.
It is possible to measure the pressure more directly, without the conformal piston. Older VihtaVouri test data (2nd ed.) was taken with the piezo sensor just in front of the case mouth, and NATO does this as well. However, the pressure is slightly lower there, and one cannot sense the pressure until the bullet's base has passed, preventing one from seeing how smooth is the initial ignition of the charge. One can also cut a small hole in the case to align with a narrow pressure sensor port, and the CIP uses this method. Either way can expose the sensor to the hot gases, leading to shorter sensor life. However, neither a conformal piston nor calibration of the brass case is required, and these approaches will likely be less expensive than SAAMI's.
Other pressure transducers can be used in place of the piezo transducer. Once more common was one using a strain gauge. Such transducers fit where the piezo unit does in modern tests.
Strain gauges can also be used in an another fashion to measure pressure. The strain gauge is glued to the surface of the barrel, over the chamber, and the "hoop" strain is measured from the small changes in resistance in the gauge's wires. The test barrel ideally has somewhat thinner walls, allowing for greater strain during firing. One must also compute an offset to compensate for the pressure being contained by the brass of the case. In a lab, this might be done hydraulically, as with piezo, but I've read it is not. The properties of cartridge brass are fairly well known, and the measurement of the brass thickness is readily done, so it's possible to compute the offset with fair accuracy. Strain gauge systems can be quite affordable; the Pressure Trace available from
RSI is marketed to the handloader.
Oehler Research also sells such systems for laboratory use.
Measuring Pressures in Handloads
In the U.S.,
H.P.White tests ammunition, and may be able to do so for home handloads. In recent years,
Western Powders advertised a modestly priced service to test handloads in their piezo rigs, but as of late 2006 that service is not listed on their web site. In Europe, the
Birmingham Proof House in the U.K. will test handloads as will
DEVA in Germany, and other national labs might as well.
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There it is, as clear as mud!