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#613406 04/02/22 08:43 PM
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eeb Offline OP
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I’m looking at a high condition 10 gauge boxlock with twist barrels. It was reproofed in 1989 with 70mm chambers, 11/2 oz shot at 850 bar. By virtue of the heavier shot charge I would think working pressure would be greater than 850 bar. Am I incorrect?

eeb #613407 04/02/22 08:54 PM
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I don’t think so. 850BAR is about 12,090, working pressure perhaps 10,750. Working pressure runs less than proof pressure. Not sure a guy should run old twist barrels up anywhere near proof load pressures.

Best,
Ted

eeb #613410 04/02/22 09:25 PM
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You are correct. I mixed up proof and working pressures. Still, 850 bar is proof for 1 1/8 oz, 1 1/4 oz and 1 1/2 oz. It seems all that matters is case length since all those loads are proofed at the same pressure.

eeb #613419 04/03/22 09:41 AM
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That 1 1/2 oz. is not traveling as fast as that 1 1/8 oz., load as a general rule. It is possible, but most loads travel faster for a lighter load and slower for a heavier load. That keeps the pressure more equal when you lack today's slower burning powders. That is the beauty of the proof system. Any gun under proof, is approved for any shell loaded to that proof service level. "Standard" CIP proof is 850 bar and has been that way since 1979. Standard proof working pressure, is up to 740 BAR (10,733 PSI). All, myself included, have been obsessing about here is what "standard CIP proof" has been for decades: standard proof "working pressure" is up to 740 BAR (10,733 PSI). No, you cannot shoot "right up to proof pressure", and is exactly why the CIP publishes "working pressure" load limits. We are making it just too hard. If your 12 gauge gun has 850 bar proof, at any shot load, which is no longer stamped on the gun anyways, then you working service pressure ought to be 740 bars, which when calculated should be about 10,733psi. If your load is under 10,733psi you are good, even better for the old stock if less, because old wood gets fragile.



Pressure can be calculated by a formula. P is pressure, F is force and A is area. Pressure equal the force divided by the area.

P=F/A

Pressure seems simple to understand as it is the result. Measure the force and calculate the area. Force is the effect of the gasses and shot acting against the barrel wall, the empty shell, the breach behind the shot and air in front of the shot, Area is the volume of space as the gases push the payload down the barrel. So as the shot goes down the barrel, the area increases and this is why we can shoot barrels not as thick as the chamber area, where the volume is smallest and the pressure can be the greatest.This is also why a slow burning powder can have lower pressure early but still achieve a good velocity. They also can have higher pressure down the barrel, compared to a faster burning powder which may peak early and drop off quicker. There are a ton of variables such as how fast a powder burn rate, how hot a primer to achieve quick ignition and full burn of powder, compress-ability of shot, steel compresses less than lead and the amount of compress-ability of the wad which will spread the pressure out a bit by absorbing energy as it collapses.

So what does this all mean. Well nothing and everything. The length of the shell is no more a certain predictor of pressure, than the length of the brass on the case. UK used long shells, pushing the same shot load used, more and better wadding. It is true there is more room for heavier load, the final limit is service pressure which being kept equal would tend to push heavier loads slower than light loads. Pigeon guns often had long chambers but the shot load was restricted by rules so the incentive was to get more and better wadding, not to cram in more shot. Speed does kill but better patterns kills as well and Black powder loads most likely were velocity limited by Black burn rate.

Now with modern powders, you can get higher velocity, with lower peak pressure because the burn rate is slower, so the area is increasing as the load goes down the barrel. That is the case until the burn rate is too slow and you get incomplete total burn of the powder and shells which vary in results form one to the other. That is why you struggle to get Longshot light loads to go much under 1250fps. If you drop the charge too much it gives you bloopers.

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eeb #613421 04/03/22 10:26 AM
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The G.E. Lewis "Magnum" 3" with 1 1/2 oz. shot 12b were introduced c. 1920 (before Olin's Progressive Burning Oval)

[Linked Image from photos.smugmug.com]

Major Sir Gerald Burrard, The Modern Shotgun, Volume II, “The Cartridge”, 1955 3rd Revised Edition reported pressures (converted from Long Tons/ Sq. Inch)
10g 2 7/8” 1 7/16 oz. 4 Dr. Eq. standard service 9,296 psi with maximum service 11,984 psi
12g 3” 1 1/2 oz. 3.57 Dr. Eq. standard service 9,632 psi with maximum service 12,320 psi

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eeb #613422 04/03/22 10:27 AM
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According to my copy of the 1989 Rules of Proof book. For Standard Proof 4 & 8 bores are Proofed at 800 bars. 10, 12 &14 bores at 850 bars, 16 bores at 900 bars and 20, 24, 28, 32 and .410 bore are proofed at 950 bars. The larger the bore the less the pressure as the gas has more room to play in and the tighter the bore the greater the pressure; a bit like squeezing a hose pipe and getting to blow off where it fits the tap. Lagopus.....

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eeb #613424 04/03/22 12:53 PM
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Thank you gentlemen for your responses

eeb #613427 04/03/22 02:29 PM
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This is an updated version of the CIP regulations converting BAR to PSI using piezo transducer derived numbers

12g 50mm, 65mm, and 70mm “Standard Proof” lead or steel Maximum Average (SERVICE) Pressure 740 BAR = 10,733 psi; Maximum Statistical Individual Pressure 850 BAR = 12,328 psi; Mean PROOF Pressure 930 BAR = 13,489 psi

Both 65 and 70 mm 16g standard is SERVICE 780 BAR or 11,313 psi; Maximum Statistical Individual Pressure 900 BAR or 13,053 psi; PROOF 980 BAR or 14,214 psi.

Both 65 and 70 mm 20g standard is SERVICE 830 BAR or 12,038 psi; MSIP 950 BAR or 13,779 psi; PROOF 1040 BAR or 15,084 psi.

eeb #613445 04/03/22 09:52 PM
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Like I said, we make way too much about service vs. proof pressure. As a personal choice I go for lower than service pressure levels in my loads. I can easily find 12 loads in the 5,500 to 7,000psi which handle almost all my shooting needs. For the 20 I bump it up to 8,000 to 8,500psi but do not get excited about 9,000psi. If you reload that gives you a ton of loads to draw from even in these times of difficult to find powders. Factory pressures for American shells are loaded to SAAMI safe levels which are higher. Unless I know for certain what they factory pressures are I just go with my re-loads or CIP approved ammo. Every shell I have loaded on my shelves are low pressure loads, with the exception of a few small batches of loads for hunting situations and they are clearly marked on the primer end. You might need to check if it is 2 1/2 or 2 3/4" at most and my pressures are so low that is not even that big of a deal. I have been much more relaxed about "service pressures" ammo for my classic doubles since I went to uniform loads. The shot size may change depending on intended use, but a 1150fps, load low pressure load, is safe in any gun I own and shoot. About as simple as it can get. I can go down to 1100fps and a few go 1200 but nothing so silly as the current rage of 1450fps ammo which seems to be the current default shell everyone seems to use.

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Originally Posted by Drew Hause
This is an updated version of the CIP regulations converting BAR to PSI using piezo transducer derived numbers

12g 50mm, 65mm, and 70mm “Standard Proof” lead or steel Maximum Average (SERVICE) Pressure 740 BAR = 10,733 psi; Maximum Statistical Individual Pressure 850 BAR = 12,328 psi; Mean PROOF Pressure 930 BAR = 13,489 psi

Both 65 and 70 mm 16g standard is SERVICE 780 BAR or 11,313 psi; Maximum Statistical Individual Pressure 900 BAR or 13,053 psi; PROOF 980 BAR or 14,214 psi.

Both 65 and 70 mm 20g standard is SERVICE 830 BAR or 12,038 psi; MSIP 950 BAR or 13,779 psi; PROOF 1040 BAR or 15,084 psi.

Very good info, Doc Drew.

Of course neither the proof nor service pressure figures are currently stamped on British shotguns. They've been replaced by STD for standard proof and SUP for superior or magnum proof.

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