doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Help needed on pressures of modern loads?

Ian: OK-that helps alot. My box of Eley Grand Prix 15/16 oz 16g loads states "...for a service pressure of at least 3 tons per sq. in. or a European proof pressure of 850 kg per sq. cm."
SO IS THIS ABOUT RIGHT??
A service pressure of 3 tons per sq. in. =European proof pressure of 850 kg per sq. cm. (BAR)= 12,328 psi proof pressure= 8960 psi service pressure= 650 BAR service pressure

You can use 14.5 for a conversion from BAR to PSI. Not sure is absolute but close enough will not get you in trouble. Thus 850BAR x 14.5 = 12,325psi "Proof" pressure. 650BAR x 14.5 = 9,425PSI Service pressure. Burrard reported on some pressure readings taken with a lead crusher & Piezo Electric gage simultaneously. Within ordinary shotgun pressures a "Workable" formula was devised. 1.5crusherP -.5 = PEP. Thus (1.5 x 3tons/sqin)-.5 = 4tons tons/sqin. Since the British do use the "Long Ton then 4 x 2240 = "Approximately" 8960PSI Service pressure for a 3 ton proofed gun.
Note the similarity here to the 650 BAR service pressure.

The problem here, as noted above, is like the old remark (George Bernard Shaw?) about the Americans and the British being two peoples separated by a common language. Thus it is with proof, service pressure, etc.

As Ian noted, although the British now express proof pressure in bars (since the late 80's), you cannot do the bar x 14.5=psi, because the Brits don't use the more modern electric transducer system, but rather the old lead crusher system--which did not accurately convert to psi. Straight from the Birmingham Proof House, which is the most reliable source I know of when it comes to questions about proof, service pressure etc on British guns:

"The equivalent transducer values that should be used for comparison with SAAMI transducer values are 740 bar service and 960 bar proof, i.e. 10,730 psi (service) and 13,920 psi (proof)."

If you look in older reloading manuals (or even some newer ones), you'll find the pressure of various reloading formulas expressed in LUP (or lead units of pressure). Those are measurements derived from the old lead crusher system, which the British still use. There's no formula to convert crusher values to transducer values, but an acceptable "ballpark" conversion is: LUP + 1,000 = psi. (Which you can also reverse, if for some reason you want to convert from psi to LUP.) If you add 1,000 to the figures Miller gives above, you come pretty close--especially to service pressure.

Mike, if your gun has 850 bar stamped on it, then it was submitted for reproof some time since the late 80's. (You may even find a 2-digit year of reproof, if you examine the barrel flats carefully.)

Regardless of the pressures reported on American factory loads, I would not use them in that gun. SAAMI service pressure for the 12ga is 11,500 psi. One reason American ammo makers don't state pressure on their shell boxes is because they load to a particular velocity level, not a pressure level. That is, they may very well change powder from one lot to another, and while the velocity of the loads will remain quite constant, the pressure will not. As long as they stay within the SAAMI standard, they don't concern themselves with pressure. CIP (British/European) shells are loaded to conform with the lower CIP pressure standards, which you can be sure will not exceed the service pressure of an 850 bar proof gun. So your choices are either CIP shells (or their American equivalent, from companies like RST and Polywad), or low pressure reloads. Because it's so easy (especially with the 12ga) to keep reloads under 9,000 psi--even for 1 1/8 oz hunting loads--that's the route taken by many people who shoot older doubles proofed to lower pressure standards.

Once again, glad you're here Larry
So the 'close but not really correct' formula should be?

A service pressure of 3 tons per sq. in.= European proof pressure of 850 kg per sq. cm. (BAR)= 13,328 psi proof pressure= 9960 psi service pressure= 650 BAR service pressure

Or asked another way:
I have this gun marked 3 Ton 850 BAR 1 1/8:
Shells at what PSI was it originally designed to shoot?

This exchange seems irrelevant as max pressure allowable 25mm from breech face is 740bars (NORMAL PRO0F) for guns with 70mm chambers, and most Euro std. game loads fall between 450 and 600 bars.

These numbers come directly from the B&P speed/pressure chart and fit the 1 bar = 14.50377 psi conversion http://www.bandpusa.com/files/db_trio.pdf
Do we therefore need to add 1000 to get the SAAMI psi equivalent?

600 bar = 8,706 psi

650 bar = 9,432 psi

700 bar = 10,157 psi

800 bar = 11,608 psi

"The Modern Shotgun" by Burrard, Appendix III, page 277.
"" A most interesting outcome of pressure measurements on shotgun cartridges using the piezoelectric gauge has been the comparsion of the maximum pressure readings with those of the lead crusher gauge. By mounting the "TWO GAUGES" on opposite sides of the same gun at one inch from the breech face ""Simultaneous"" readings on the same cartridge were possible. As had been known for some time, the crusher readings were les than those given by the piezoelectric gauge due to the time lag in the plastic deformation of the lead-the piezoelectric gauge piston only moves about 1/100,000 inch when subjected to 2.5 tons per sq in and is thus free from this error. Over the ranges of pressures usually encountered at the one inch position in shotguns the readings given by the two gauges are related by the equation;
P = 1.5L - 0.5 tons per sq in
Where;
P = the maximum pressure indicated by the piezoelectric gauge
L = the maximum pressure indicated by the lead crusher gauge ""

Take it for whatever it appears to be worth to you, but do note the formula was "Derived from actual Simultaneous readings by both gauges recording the exact same pressures". Also note it is a conversion of Lead crusher pressures to tranducer pressures.
From the formula;
2 tons (4480) = 5600psi (386BAR)
2.5 tons (5600) = 7280psi (502BAR)
3 tons (6720) = 8960psi (618BAR)
3.5 tons (7840) = 10640psi (734BAR)
4 tons (8960) = 12320psi (850BAR)
As can be seen this formula is not totally linear but would be a part of a curve, nor does adding 1,000lbs get very close. I have nothing to indicate it would be accurate much beyond this limited range from which it was derived,but then this is the range in which we are primarily concerned. It would not of course be aplicable to CUPs as used for Rifles, & not sure it would apply to US LUPs as actual measurement & calibration may not be identical. This IMO should be quite adequate for any British gun with proof marks given in Tons which denote the service pressure.
With all due respect to the figures Larry has posted from the Birmingham proof house I think they are not stating the same situation as this formula. The 14.5 conversion is of course a mathematical conversion for measurements taken by the same method, thus recording the same pressure, not for conversion from one system to another. I may be wrong but I believe "Continental" BARs are tranducer figures.
Note if the gun was originally proofed @ 3tons & subquently given a re-proof of 850Bar for 650BAR service loads, both given in crusher measurements, & I have no reason to doubt that what Larry stated is totally true, it has in fact been re-proofed to a higher pressure level than the original 3ton proof. The formula I have given is in fact more conservative & I believe would be a safer course on older guns not bearing a re-proof. Also I believe the still older British "Nitro" 1 1/8oz proof is essentially the same as the 3ton proof.

And the plot thickens further.
My box of Gamebore Traditional Game 16g 28gm (1 oz.) which Gamebore tells me runs 1260 fps at 8122 psi (well below the 8960 service pressure) has written:
"These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4" or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
67 mm case length: Guns with a chamber length of 2 1/2" or longer, nitro proofed to a service pressure of 3 tons per square inch (850 kg per square cm.)"

Looks like there is a different modern 'standard' for 2 1/2" and 2 3/4", but as discussed in a previous thread, 2 1/2" chambers are indeed proofed at 850 bar.

Thanks to all for the education, time and insight.

I think I am learning enough now to be dangerous. If someone has the time please review below and let me know if I am getting a little brighter on understanding this can of worms.

On a very simplistic basis - it is very difficult to accurately convert british proofs to PSI, because the U.S. and England (proof houses) use different standards for measuring pressure. It sounds like there are some conversion factors that some of you feel are reasonably accurate (I haven't digested if they are essentially the same)

If you have a 2 1/2" chambered 12 ga gun that is proofed for:

Nitro Proof for 1 1/8 oz (proof mark for 1904 to 1924?)
3 tons (proof mark for 1924 to 1985 or so?), or
850 BAR (post 1985 or so) ALL THREE BEING ABOUT THE SAME PRESSURE TEST - THEN KEEP SHELL PRESSURE BELOW 8500 - some felt 9000. To do this you need to use CIP, Poly Wad or RST shells. Fortunately that is all I have ever used.

Question - if a 16 ga gun has proof marks for 1oz Nitro Proof and 3 tons it was probably also reproofed? Again use above shell selection.

It seems like the guage would impact pressure, and a 16 ga at 3 tons and a 12 ga at 3 tons would need to take different pressures, or the 16 ga would need thicker or stronger barrels?

Pressure does not equal recoil, because recoil is measured in foot pounds.

U.S. manufacturers are not as concerned with pressure because they are trying to meet a different standard i.e. FPS, and change powders from time to time resulting in different pressures, but the same FPS.

Depending on the age of your English gun and proof, there could be times that some standard U.S. ammo might work - at lease based on tests run by Amburst for 16 ga site.

It sounds like there is a mimimum standard the Brits are trying to meet for all guns. For that matter same goes for U.S. guns, and maybe guns imported for use in U.S.?

Mike: you've got the bottom line figured out (while us pea brains try to figure out the conversion numbers )-
keep the pressures below 8500 or so, which is alot easier to do with a 12 than 16 or 20, and you'll be fine, using a gun that is in proof. And BTW: 7/8 oz. 16g loads are delightful.