doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Question for Drew...

That 12ga service pressure, considering that psi back then was actually LUP, would be quite close to the 10,730 psi service pressure of the current "standard proof" CIP guns. Pretty much all the new CIP guns we see in this country, however, are superior (old magnum) proof.

larry;
I seriously doubt that these pressures stated in this Army Ordnance publication were LUP pressures. These pressures are stated to be from the 1925 British Rules of Proof. Consider the the mean working pressure of Factory Loaded shells in 12ga for 2 5/8", 2 3/4" & 3" are given respectively as 4�, 4 3/4 & 5 tons.
These are not pressures we normally associate with LUP pressures of that era but are much more in line with actual PSI. These as stated are the Service pressure for the ammunition which is slightly lower than the mean working pressure of the gun itself. The mean Proof Pressure of these three lengths of 12ga shells are 6.1 (13,700), 7.1 (15,900) & 7.75 (17,300) tons.
"IF" these are LUP pressures the Brits weren't loading to lighter pressures than us, We were loading lighter than they.
Although admittedly not in common usage at the time, transducers were around by 1925 I believe & apparently this Army Ordnance publication made use of them.

Miller, I tried a longer post but got some sort of "internal server error" that ate it. Shorter version: American Rifleman article, 1931, "Standard Shotgun Pressure Barrels": describes the crusher method and shows the barrels in use by everyone in the industry. All crusher. As for the Brits, they were still using crushers at the time they switched from the "tons" proofmarks" to bars, in the 1980's. Confirmed this with the Birmingham Proofhouse when I misinterpreted the 850 bar proof pressure as being a transducer value. No, they told me. That's a crusher value. They converted to transducers in the early to mid-90's, but kept the same crusher-derived 850 bar proofmark. Another attempt on the part of those perfidious Brits to confuse us.

Yes, this is all well & true, but Do Tell Me. when the British started marking their guns for the service load in tons they were proofed for what was the markings on the 2 5/8", 2 3/4" & 3" chambered guns. It surely wasn't anywhere near 4� tons, 4 3/4 tons & 5 tons. These numbers just do not correlate with the LUP pressures of the 1925 era. These guns would all have fallen into the 3 ton to 4 ton range Not 4� to 5 ton range.

Miller, can't help with exactly what you're looking for. However, I'll offer this, directly from Roger Lees, proofmaster when the change was made from the 1925 rules to the 1954 ("tons") rules:

"The proof loads now set down in writing (in the 54 rules) are almost in every instance the proof loading in use under the 1925 rules. In general it may be said that under the new Rules of Proof no arm will receive a more severe proof than hitherto."

Larry;
Do note that the mean proof pressure given by Mr Goddard for the 2 5/8" chambering was listed @ 13,700 (He did not put an actual unit on it). 13,700 would more closely relate to 950 BAR than 850, & recall this was stated to have been from the 1925 British Rules of Proof. 850 BAR would equate to about 12,300pounds or some 1400 lower than the 13,700 he gives. When looking at the numbers the "Only Logical" conclusion is that in preparing this article for the Army Ordnance Mr Goddard by some means converted the pressures to actual PSI & did not use LUP. Also note the article was printed in 1934 so the technology for the conversion did not have to actually exist in 1925.

Miller: The Long Tons to PSI appears to be the simple mathematical conversion rather than (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI; and we know even that is an estimate.

6.1 X 2240 = 13,664
6.1 X 1.5 = 9.15 - .5 = 8.65 X 2240 = 19,376 ???

For comparison from "Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" by Wallace H Coxe; E.I. du Pont de Nemours & Co., 1931



Charge was 1 1/4 oz. Dram eq. was not specified but was likely near Maximum. Pressure is expressed in Long Tons.
Using the estimated Lead Crusher Pressure (Cp) conversion to PSI (pound force per square inch) (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI.
Ballistite maximum pressure at 1� was 4.9 Long Tons = 15,344 psi
Schultze at 1 1/2� was 4.5 Long Tons = 14,000 psi

Again per the Birmingham Proofhouse, 850 bar is a crusher measurement. 960 bar is the equivalent transducer measurement. Converted to psi that's 13,920. Looks like we're coming pretty close.

Drew;
When this chart was published in the American Rifleman (minus the Ballistite Curve) in the 1950's they stated the load was a 3 dram or equivalent charge with 1� oz shot. Transducers for measuring actual PSI was available by the 30's & it is highly possible that DuPont availed themselves of their use. We simply cannot say for a certainty that these pressures are LUP's.
The main thing though is the British pressures that Mr Goddard shows as being 4� to is the British load that we normally associate as a 3 ton/ 1 1/8 oz proof. All I am doing is citing the two sets of pressures & noting they do not match. If however Mr goddard did in fact cite actual PSI rather than LUP they match quite nicely. Transducers had been built by that point in time, so it is a possibility.
When the British worked out that LUP (tons) x 1.5 -.5 (ton) = PSI, they did so by firing loads through a pressure barrel equipped with both sensors so both measures were taken simultaneously. They did not state this to be an exact formula but that within the range of shotgun pressures that it was close enough for practical use. By this conversion a 3 Ton LUP converts to 4 Ton PSI. Mr goddard showed 4� ton. Close enough for me to believe he was showing actual PSI's. Nuff Said, unless someone can actually show me how mathematically that 3 LUP = 4� LUP.

Miller: I don't question that transducers were known in the late 1920's but I do question whether they could be used for pressure and time measurement at that time. Beginning around the start of WWII, the oscilloscope had been developed which could be used to show both an electrical pulse and a time line when hooked up to a piezeo crystal, but I don't know how the output of a crystal transducer would have been displayed before that time.