British System of Chamber Pressure Measurement |
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Honkytonk
Senior Member Joined: December 30 2017 Location: Brandon Mb Status: Offline Points: 4763 |
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Goosic. Lots of people don't realize that there are a lot of hp eating parts between the flywheel and the pavement! Then you have to get that remaining hp to launch your vehicle. My 1990 Chevy Silverado two wheel drive has a Procharger blower on it for an engine hp of 450. About 300 of that goes up in tire smoke rather than make me go fast when I launch!!! Fun to drive, but I think my wife's 1.6 turbo all wheel drive Hyundai Kona would beat it in the 1/4 mile!
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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I found the information on the .303 Proof cartridge pressures. This was from a web site titled “British Military Small Ams Ammo”. I had booked marked this site a few years back, it is not showing the actual web address.
Proof Mk III cartridge pressure was set at 25 Tons per square inch (56,000 psi) average pressure, by the time of WWII. This is based on the British system of measuring chamber pressure by a base copper crusher as described in the OP with an oiled cartridge. Using the 4/5 ratio between base pressure and CUP, we can estimate the Proof pressure for the .303 as i.e 5/4 x 56000 = 70,000 CUP. But that’s just an estimate, and the 4/5 ratio is based on the .30-06 cartridge. Before you all get excited, don’t even think about loading up some “magnum” .303 loads, stick to the published data. Frankly, I’m a bit surprised that there is not a larger margin on proof load over the service load (25/18.5 = 1.35). This is a lot of pressure just a few inches from your face. Designers of bridges use a factor of safety of 5 over the expected service loads. In aircraft structure, we use a factor of safety of 1.5 on the maximum load ever expected to occur once in the lifetime in the fleet of aircraft. And a new design is always tested to this “Ultimate” Load to ensure failure does not occur. We don’t know what the failure pressure would be in the No. 4, but detectable permanent deformation should not occur at the Proof load. It would be effective in identifying significant manufacturing defects, but does little to ensure future fatigue failures cannot occur. |
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The Armourer
Senior Member Joined: June 23 2019 Location: Y Felinhelli Status: Offline Points: 1246 |
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Ishapore tested 303 with a 24 tonnes PSI breech presssure.
It is at a slight tangent, but when Ishapore changed the steel specification from that specified by the 'Brits' and then started production of the 2A rifle they found that the action distorted and the bolt jammed when proof tested with an oiled round.
They changed back to the old steel specification but continued to test only with the 'dry' round. An article from the Proof-Master of the Ishapore factory Extract from “Gun Digest 33rd Anniversary 1979
Deluxe Edition” Article Author : Mr A G Harrison Qualification : Former ‘Proof Master’ of the ‘Rifle Factory
Proof House, Ishapore, India’ From 1908 to 1950 all military bolt action rifles made at
Ishapore were proof tested with a dry-round, followed with by an oiled proof
round. The proof cartridge was loaded to 24 tons psi breech pressure, or 15%
higher than the service pressure. In 1950 (after the departure, in 1949, of
India from British control) the material for the rifle bodies was altered
from an EN steel to SWES 48 steel with the recoil shoulder and cam recesses
being heat treated. With this change the rifle receivers distorted when oiled
proof cartridges were fired. This was discovered when hard and sometimes
impossible bolt retraction was experienced. Large quantities of rifles were
rejected. To avoid rejections the authorities ordered discontinuance
of the oiled proof round. Therefore from 1950 to the end of SMLE production,
rifles made at Ishapore were proof tested with one dry proof only, although the
specification still called for both dry and oiled proof. All bolts and bolt
heads issued as spares were always proofed with a dry proof round only. A bolt action rifle similar to the SMLE MkIII*, modified to
fire the 7.62mm NATO cartridge, was produced at Ishapore, first in February
1965. The receivers were made of SWES 48 steel (as per the SMLE MkIII*) and
with the NATO proof cartridge the receivers were found to distort with both the
dry and oiled proof round. The material was changed back to the EN steel so now
the rifles stand up better to dry and oiled proof. After passing proof the
barrels are impressed with the Indian national proof stamp. The bolt handles
and bolt head claws are struck with the crossed flags only. |
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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Pulled this off the same web site for the 7.62 NATO. Intended for the L1A1 but may also have been used to proof the L8 and L42, don’t know.
27 tsi = 60,480 psi 29 tsi = 64,960 psi These are some high pressures. Peak pressure estimated as 5/4 x the above average pressures, assuming the same method of measuring pressure was used as described in the OP. It should have been since the conversions were done in the 1960’s. Something else to consider, was the case oiled when performing the proof pressure of the rifle? It would seem unnecessary, as it significantly increases bolt thrust. The oiled case was done to facilitate pressure measurement using the copper crusher test rig. This is a significant question as it relates to what the actual proof loading would be on the action of the rifle. Either way, the chamber sees the same pressure, but bolt thrust increases considerably by oiling the case. My guess, and it’s just a guess, that the case would not be oiled for the proof test of the rifle. |
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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To follow up on the question of how the No. 4 rifles were proof tested during production, the following is an extract from Reynolds book “The Lee Enfield Rifle” on page 177:
Each barrel, with action assembled, had to withstand two proof rounds, the proof charge being designed to give a pressure of 26 tons per square inch, which was approximately 25 percent in excess of that of the normal Service cartridge. The first was fired dry for proof of the barrel, and the case of the second round was lightly oiled to prove the action, especially the bolt head. If satisfactory after the examination the barrel, body and bolt head were marked with the Government proof mark. Components made for spares were proofed accordingly. We now know how the pressure measurements were made, using a copper crusher measuring axial pressure from an oiled cartridge case. But no formula to convert the base copper crusher units of pressure with the American Copper Units of Pressure (CUP). The article suggests the ratio of base pressure to radial pressure is approx 4/5 for the .30-06, and it is dependent on the cartridge. |
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The Armourer
Senior Member Joined: June 23 2019 Location: Y Felinhelli Status: Offline Points: 1246 |
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As I linked to earlier, the Ishapore Proof house tested the No1 MkIII with proof rounds at 24 tons PSI |
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Goosic
Senior Member Joined: September 12 2017 Location: Phoenix Arizona Status: Offline Points: 8760 |
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The Armourer.
For me,it does not add or detract nor confuse me in the slightest. It just solidifies what I already know.
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Goosic
Senior Member Joined: September 12 2017 Location: Phoenix Arizona Status: Offline Points: 8760 |
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So,according to what you read. The proof of 26.0 Tons Per Square Inch, (TPSI) would calculate out to 57,320.188172 CUP. 25% in excess of the normal service cartridge gives you a calculated 56,250 CUP. The rated 18.5 TPSI stamped on the barrel only calculates out to 40,785.518507 CUP however. 20.5 TPSI calculates to 45,194 CUP ,the working pressures for the service cartridge which is two tons per square inch over the 18.5 TPSI stamped on the rifle when it is sold into.the civilian market.
Calculations using the Short Ton metric system. The Imperial Long Ton is 35 pounds over the short ton and calculations still fall within the CUP/PSI standards as per the chart The Armourer has provided.
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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I think I found the missing link here. I came across another article on measuring chamber pressure and the description of the British base crusher method states that it produces between 10 to 20% lower pressure than the Radial crusher method. The article in the OP stated this was approx 4/5 ratio (base to radial), which is 20%. It may well be cartridge dependent.
If I now use the 18.5 tsi stamped on the barrels of rifles sold for commercial sale converted to CUP would be: 18.5 x 2240 x 1.1 = 45,440 CUP SAAMI list maximum pressure for the .303 as 45,000 CUP. C.I.P lists the .303 maximum pressure as 46,000 CUP. Now it’s getting closer. Using this conversion, it seems to be closer to other data. Converting the 26 tsi as the proof cartridge given in Reynolds book, we get: 26 x 2240 x 1.1 = 64,064 CUP |
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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Gossic, I have found several articles that state the British system of measuring pressure uses Long Tons per square inch (tsi). 2240 lbs exactly to the long ton.
Also, the estimate to convert the British base (axial) method of pressure measurement to the American and European radial pressure measurement: British Base Pressure x 1.1 = CUP. Reynolds statement that the 26 tsi proof cartridge being 25% higher pressure than the Service cartridge seems to be a low estimate, it’s more like 30%. I’ve found pressures of the Mk 7 Service cartridge stated in various sources running between 18.5 and 19.5 tsi. This would be 45,584 CUP and 48,048 CUP. |
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Goosic
Senior Member Joined: September 12 2017 Location: Phoenix Arizona Status: Offline Points: 8760 |
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Reynolds statement is what I find intriguing. Using the Imperial Long Ton, 2240 and British Base Pressure 1.1 along with the stated 26.0 tons proof test you get a CUP of 64,064 and that is a Reynolds stated 25% in excess of the normal 303 service cartridge pressure, which is calculated to 48,048,according to the above used calculation. The 48,048 working 303 service cartridge pressure can then be safely calculated to 19.5 TPSI. Which is 3048 pounds per square inch above the rated 18.5 TPSI/45,000 CUP. The 7.62mm L39A1/L42A1 rifles are proofed to 19.5 TPSI
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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So, what does this all mean? I’m sure many of you didn’t need to know any of this, it was all because I could not relate the pressure values listed in our reference books on the .303 cartridge pressures to modern reloading manuals. The answer is that we cannot precisely relate the two. The tons per square inch pressures have been confused with CUP, and they are not the same. Some have erroneously converted tons to pounds by multiplying by 2000 (US Short Ton).
So, my own match load of 40.0 gr Varget with 174 SMK I can estimate the chamber pressure by interpolating the Hodgdon published data for this bullet at 41,300 CUP. My best estimate of the Mk 7 Service cartridge chamber pressure is 45,584 to 48,048 CUP. I now know that my loads are indeed significantly less than the Service round. Service ammunition was loaded to a velocity specification. Several versions of Cordite and NC powder was used, so maximum average pressures will vary depending on the propellant used. I chrono’ed my 40 gr Varget Match load and also 1951 DAC Canadian Mk 7z ball ammo. Velocity at approx 25 feet from the muzzle for my match loads ran an average of 2368 fps compared to the Mk 7z at 2412 fps. That Canadian ammo was right on spec. To understand the differences the bullet made to the velocity (174 gr SMK vs 174 gr Mk 7 Ball), I substituted the Mk 7 bullets for SMK by pulling the bullets, neck sizing, returning the original powder, and seating the SMK. The result was a mean Velocity of 2476 fps, an increase of 64 fps with the SMK. This I attribute to the smaller bearing surface of the boat tail bullet and .311 vs .312 bullet diameter. Pulled Mk 7 bullets loaded with 40.0 gr Varget ran at 2341 fps, about 70 fps slower than the DAC ‘51 Mk 7z service cartridge. All this gave me good confidence that 40.0 Varget is indeed a mild load for the .303 and gives very good accuracy with either the 174 gr SMK or Hornady 174 BTHP Match. This is important if you want to shoot 2000 rounds a year and want to minimize wear on the action and the barrel throat. |
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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I believe you are correct Goosic. These ratings we have (SAAMI, CIP) May be based on older actions, such as the Long Lee and SMLE. Reynolds states the Mk VI Service cartridge Introduced in 1904 was 17.5 tsi, which would be 43,120 CUP. And Mk VII Service cartridge between 18.5 and 19.5 tsi. They must have struck a compromise in setting the 45,000 CUP limit. One clarification, I was not able to specifically find what the No. 4 actions converted to 7.62 x 51 were proofed to. I did find that early British 7.62 Ball ammo was loaded to 21 to 22 tsi, somewhat higher than Mk 7 ball. Of course, you can’t get more velocity without more pressure. On top of this the diameter of the base of the 7.62 chamber is also slightly larger than the .303 giving higher bolt thrust.
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Goosic
Senior Member Joined: September 12 2017 Location: Phoenix Arizona Status: Offline Points: 8760 |
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Published data for factory/store bought made 174 grn 308 Win ammunition loaded with 41.7 grns of IMR4064 has a 39,177 CUP rating. Using the Imp Long ton British Base pressure math you get 2240×15.9×1.1= 39,177 working pressure. 8871 pounds per square inch lower in difference from the 303 service cartridge pressure.
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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Yes, for that particular load, which is quite light for .308 Win.
Hodgdon data gives 41,200 psi Piezo pressure with 41.5 gr 4064 and 175 gr SMK as the minimum charge and very close to the load you cited. Max charge for that powder/bullet is listed as 45.6 gr at 59,500 psi piezo. I would not shoot that max load in my DCRA conversions, but minimum .308 loads are perfectly fine.
Max SAAMI pressure for .308 is 52,000 CUP, 62,000 psi piezo from what I can find. Then, there is the argument that 7.62 chambers are not the same as .308 chambers giving different pressures for identical cartridges.... |
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britrifles
Senior Member Joined: February 03 2018 Location: Atlanta, GA Status: Online Points: 6512 |
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This has solved a great mystery for me. I’m sure for most of our forum members, this is a boring and irrelevant subject. But alas, I confess, I’m an engineer and I believe facts and data matter. Feel free to bash engineers, I have a thick skin.
Goosic, we should not expect all commercial .308/7.62 ammo to be loaded to the chamber pressures of the load you referenced. I agree that this load is perfect safe in our 7.62 No. 4 conversions. In fact, most, if not all, 7.62 x 51 will be safe in No. 4 rifles proofed for that cartridge. It turns out that 40.0 gr Varget with 174 gr SMK in the .303 and 168 or 175 gr SMK in the .308 makes a very accurate load in my No. 4 rifles. The reason I don’t push to the higher .308 velocities/pressures is that I want to minimize wear and tear. This is the beauty of handloading, complete control of these variables. When I first started reloading for my DCRA conversion rifle I was aware of the reports of short range accuracy problems that were in part blamed on the higher 7.62 pressures as compared to .303. So I loaded to minimum .308 load tables and presto, very good accuracy. Shown below from the same website that states the MK VII .303 is loaded to 19.5 tsi: |
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