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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?

Maybe we ought to make this thread, or one like, it a sticky. One of the most asked and often misunderstood question, is what load to shoot in my gun? A gun in proof, which has been inspected by a knowledgeable gunsmith, should be safe shooting a “service pressure” load of 740 bars which is 10,732.79 psi. For additional margin of safety you can always go lower, as I do. A Black Powder proofed gun gets loads comparable to what was original to it, and those are 4,500-5,500psi. Or if you can load them, real black powder loads which always draws a lot of attention.

Drew has posted a lot of very detailed information on this subject, along with pressures for period loads for the 1890-1900 timeframe. Look at his website to see real numbers. Look at the CIP link I posted, for real numbers. I posted the formula to calculate bars to psi. Go to Hodgdon or Alliant to find real pressure numbers for any reloads. Any factory CIP approved load will be at or below Service pressure. But the bottom line is normal service pressure is 10,732.79 psi. That goes for 2 1/2”, 2 3/4” or 3” if proofed at 850 bars. I also think 2” are now proofed at the same but check it on the CIP to be certain.

We know shooting 2 3/4” shells in 2 1/2” chambers does increase pressure some. Most reports have bee 1,000psi or less. You decide what to do about that. I cut hulls down to 2 1/2” but if the pressure was 5-7,000, in a 10,000 gun, that is over caution. I still do it.

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.

The G.E. Lewis "Magnum" 3" with 1 1/2 oz. shot 12b were introduced c. 1920 (before Olin's Progressive Burning Oval)



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

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.....

I just thought for completes I would photograph a couple of cartridge box tops containing information about the contents inside. The top box flap is Hull cartridge company this being my go to for all my nitro proof 2 1/2 chamber guns the other is if I want something a little faster the Lyalvale for clay though still for all my 2 1/2 chamber guns. There is no pressure quoted because here we only need to know the gauge chamber length and shot weight, so if your gun is in good condition and in proof for the type of propellant in this case Nitro the cartridge can be used in any gun irrespective of age in my case gun's made from 1860 to 1950 all my other guns are 2 3/4 chambers.

Jon, there's an issue with the bars figures you use. Back when the Brits stamped their guns 850s bar proof, the service pressure figure they used was 650 bars service pressure. BOTH of those figures were derived by the old lead crusher system. You correctly converted 650 bars crusher to 740 bars transducer so that it could then be converted to psi using the mathematical formula. 850 bar is also a crusher-derived figure. Thus, it also needs to be converted. The proof pressure for an 850 bar gun is 960 bars transducer, which converts to 13,920 psi proof pressure. That's straight from Roger Hancox, the former Birmingham proof master. But as you point out, the 10,730 psi service pressure is the number that's really important as we work up reloads for guns marked 850 bar, or the current "STD".

Jon, sometimes I think that our cousins across the Pond do it just to confuse us! The shotshell box is yet another example. They don't express velocity the way we do. Ours is quite consistent: it's measured at 3 feet from the muzzle. Theirs may be either a)"True" velocity, measured at the muzzle; or b)"Observed" velocity, which is the average over 20 yards; or, most recently, c)"Mean"velocity, taken at 2.5 meters from the muzzle. True velocity would lead Americans to believe that British shotshells are faster than American shells. Observed velocity, which is 1070 fps for a "standard" load, would lead us to believe that their shells are a good bit slower than ours. I'm guessing the Lyalvale shotshell box above carries a true velocity reading. The observed velocity standard for a high velocity load is 1120 fps. That equates to an American load with a listed velocity of 1235 fps. 1400 fps must be a reading taken at the muzzle.