doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Thin barrels passing London proof

The chart seems to show there is no difference after 2" which is within the chamber.

"Chamber pressure" is 1" from the breech

Wallace H. Coxe, in �Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics� published by E.I. du Pont de Nemours & Co., 1927, �Measurement of Pressures�
The common method of taking pressures in small arms in this country is known as the Radial Pressure system. A housing is built around the barrel, and a hole drilled through the housing and barrel into the chamber at a distance of 1 inch from the breech and at right angles to the axis of the bore. The hole is then bushed and drilled to a uniform diameter of 0.2250 inch. Then a piston is made the length of the piston hole and 0.2250 inch in diameter. Next the piston hole is lapped to permit the piston to fit snugly without either sticking or getting out of alignment.
In firing the gauge, the piston is inserted and seated, then a lead crusher cylinder is placed on the head of the piston and held firmly in place by a screw and anvil attachment built into the housing. When the cartridge is fired, a portion of the same gas pressure that pushes the bullet through the barrel drives the piston against the lead cylinder and compresses it.
The length of the lead crusher cylinder after compression is naturally less than before the shot was fired and the difference between the original length and the length after compression therefore represents the amount of pressure which has acted upon the lead. Thee exact pressure is read from a table giving a pressure reading for every remaining length reading and commonly called a Tarage Table.
Pressures that are determined at ballistic laboratories are merely relative values and are not absolute values.

Modern piezo transducers can be placed every inch if so desired

I don't believe London proof has existed for some time. And Birmingham proof has been increased considerably. Two proof rounds are fired in each barrel.
Dig had an article in DGJ that pointed out a model of Cogswell and Harrison was gathering a reputation for failing reproof, the article had photos of an old gun with a cracked frame.
I suspect that many of these same guns would be just fine if used with the loads they were designed for, but, can offer no proof.
The only double guns I ever had that had .030 of muzzle wall out to the 9" from breech measurement were Darnes. My Italian guns left the factory at about .025 in that area of measure.
I'm not interested in doubles at .020 or less, in the same area. I use my guns.



Best,
Ted

Ted: I couldn't find the previous thread discussing the inexplicable failures at the Birmingham Proof House, but this was clarified to mean ONE round that develops the desired pressures at both 1" and 6.38"

2006 British Rules of Proof
http://www.gunproof.com/Proof_Memoranda/RULESOFP.PDF
Part IV, �The Proof Load�, Number 27, Part A
Calls for a load of 30% over mean service pressure at a point 17mm or 25mm (about 1�), and at a point 162mm (6.38�) from the breech face a load of 30% over mean service pressure (at that point as determined by the Proof House).

"Mean Service Pressure" is probably 650 bars (9,427 psi) or maybe it is �Maximal Average (Service) Pressure� 740 BAR = 10,733 PSI.
�Mean Proof Pressure� is 930 BAR = 13,488 PSI, or maybe 960 bar = 13,924 psi.
9,427 x 1.3 = 12,255
10,733 x 1.3 = 13,953?

What the pressure is suppose to be at 6" is a secret, as is the powder in the proof load, and I've given up

My point was (obviously lost or not conveyed well enough) was that guns pass proof with low MWT.

Not that I recommend using them that low.

For the record I use my guns too. Year round........

Cheers!

That is indeed what Drew's graph shows: No discernible crossing point. The often-cited Dupont graph (from 1933) shows a crossing point at about 2 1/2", where the lower pressure loads overtake the higher pressure loads. That same graph shows that by the time they reach a distance of 7", there's a spread of about 500 psi between high and low. And by the time they reach 10", there's no significant difference.

so, rather than quibble about exactly where maximum pressure is, how much barrel wall do youse guys like in front of the chambers, or do you care? i have seen enough guns with barrel wall bulges in front of chambers to consider barrel wall thickness in front of chambers to be an even more important safety issue than minimum barrel wall thickness...

Well Larry, the point is "IF" the same ballistics are met, if the max pressure is lowered it "Will" be made up somewhere else. The laws of physics have not been changed. As max pressure is reached inside the chamber the "Make-Up" occurs over the rest of the barrel's length, so the difference in pressure is much slighter than is the Max.

Bottom line is though that shooting a "Low Pressure" (IE low max chamber pressure) load ii no way makes t Easier on the barrel at a point some 18" down the bore. The pressure there will in fact be very sightly higher, though mostly insignificant. Just don't be fooled into thinking that shooting low pressure loads are of benefit anywhere other than in the chamber itself for they are not.

With all due respect to Drew, the problem is the pics he posted are simply not detailed enough to show that slight cross over. It is simply impossible to get the same ballistics & have that much difference in max pressure & it not be compensated for else where.

I high resolution scanned this from my copy of the 1927 "Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" by Wallace H Coxe.
It doesn't tell us everything, but probably tells us something about the pressure curves of the powders
Ballistite - Dense Smokeless
Schultze and DuPont both Bulk Smokeless
FFFg BP
DuPont Oval the first "Progressive Burning" Smokeless

What you say is true, Miller. But we know for a fact that we can have loads that differ by a few thousand psi PEAK pressure. It'd be interesting to chart loads with those very significant differences vs those that only differ by 1,000 psi or so peak pressure. But what the old Dupont charts show--like Drew's, and like the 1933 chart I'm looking at--is that by the time you reach 6" from the breech, the difference is already insignificant--in the range of 500 psi.

So it seems to me that it depends on where you're worried about pressure, because from 6" or so on, the curves merge together even closer. As far as pressure is concerned, beyond that point there's no reason to concern yourself at all--at least as far as the difference between loads is concerned. If your barrel is going to fail due to pressure at that traditional 9" measuring point, I doubt that the lowest pressure load at about 3,600 psi would save it from happening with the highest pressure one at 4,000.