An important but 9 year old thread revived by Bill, then his comment deleted.

I went through each page and unfortunately can not edit nor restore the images.

I'll address Bruce's question in the last post from 2014 then stick to wall thickness measurements

To clarify the issue of barrel strength and wall thickness, tensile strength is only a part of the equation for estimating bursting pressure. There are several burst formula.
All refer to a pipe capped at both ends with a static pressure (a pressure cylinder). UNOBSTRUCTED shotgun barrels are not designed to be pressure vessels as one end is open and the pressure rises and falls quickly SO (according to several mechanical and metallurgical engineers with whom I spoke including Eldon) NONE OF THE FORMULA WORK.

Barlow's formula P=2 S t / D
P=Bursting pressure in psi.
S=Tensile strength of material in tube wall.
t=Wall thickness in inches.
D=Outside diameter in inches.

Burrard used the Alger Burst Formula
Burst pressure = Ultimate tensile strength x 3(OD – ID) / OD + 2xID

The Hoop Stress Formula doesn't reliably predict shotgun barrel failure either
Shotgun barrels are "thin wall cylinders"
σ = pr/t
p= pressure; r is the inside radius; t is the wall thickness

Bottom line is we can't estimate bursting pressure in a properly designed and fabricated barrel, free from internal or external defects.
We do know that it took Sherman Bell >30,000 psi to burst both Parker D3 Damascus and Vulcan Steel barrels


Here's a long thread from last year about measuring wall thickness which has lots of end-of-the-chamber numbers WHICH ARE MUCH MORE CRITICAL than MWT in the distal 1/3 of the barrel.
https://www.doublegunshop.com/forums...&Number=615160

More thoughts regarding barrel evaluation & load recommendations
https://docs.google.com/document/d/1...vwLYc-kGA/edit


We now have LOTS more data regarding the actual loads used in our vintage doubles, and Parker Bros. and the other maker's load recommendations. There are several pressure-distance curves also.
https://docs.google.com/document/d/1...aAfUOZEFU/edit

BTW Parker Bros. documented as early as 1893 that they proved barrels in-house.

A Parker Service and Proof Load table was published in the 1930s and reproduced in the The Parker Story p. 515. 12g 2 3/4” shell Service Pressure is 10,500 psi. Definitive proof used 7.53 Drams Black Powder and 2 oz. shot with a pressure of 15,900 psi. The pressure was no doubt measured using LUP and modern transducer values would be 10-14% higher, or more than 17,500 psi.

LTC Calvin Goddard reported the same numbers in “Army Ordnance”, 1934. He wrote that Parker followed the SAAMI standards of that period: 13,700 psi proof, 9500 psi service for 2 5/8” chamber; 15,900 psi proof, 10,500 psi service for 2 3/4” chamber (by LUP) + 10-14% for modern transducer measurement.

To paraphrase Bill’s post, excessive bore diameter has little relevance without knowing the barrel wall thickness.





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re: bore measurements
After the proof house revisions of 1887, the British Proof House began using fractional bore

https://photos.smugmug.com/Proof-Mar...h%20Bore-L.jpg

So a British 12 bore (and marked '12') could be .729" - .739". If now .740" or > .010" larger than at proof, the barrel is considered out of proof.
The relevance to our U.S. maker's vintage doubles is that a bore greater than expected, esp. with a "mirror" bore, has very likely been honed; making accurate measurements of wall thickness from breech to muzzle imperative.

1954 - 1989 the bore in inches was marked and chamber length in mm, but still with the bore in a diamond.
The bore in inches was dropped in 1989 and replaced with mm.

We also know the factory chamber lengths (which varied over time) of our vintage doubles, and if longer than expected then measurement of the end-of-chamber wall thickness is critical.

Some of the CIP shotshell pressure standards have changed since the 2012 post, and are relevant in that lots of CIP member nation loads are now available
The Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives
https://bobp.cip-bobp.org/en/tdcc_pu...idge_type_id=7

https://photos.smugmug.com/Shotshell...evision-XL.png

12g 50mm, 60mm, 65mm, and 70mm “Standard Proof” lead or steel (limited to no larger than 3.25 mm / U.S. No. 4s and max. fps 1,300)
Numbers are transducer BAR converted to PSI.
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
(The original 1984 chart, which was revised in 1990, indicates 12g PROOF 960 BAR = 13,924 psi. The number was revised to 930 BAR)

12g 3” & 3 1/2” “High performance/Superior Proof”
Maximum Average (Service) Pressure 1050 BAR = 15,229 psi
Maximum statistical individual pressure 1200 BAR = 17,405 psi
Magnum proof 1320 BAR = 19,145 psi

12g STEEL regulations: the barrels should carry the High Performance Steel Fleur de Lys stamp and be marked “Steel Shot”

Both 65 and 70 mm 16g standard is SERVICE 780 BAR or 11,313 psi;
MSIP 900 BAR or 13,053 psi;
PROOF 980 BAR or 14,214 psi.
(The original 1984 chart indicates 16g PROOF 1020 BAR = 14,794 psi.
The number was revised to 980 BAR in 2006.)

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.
(The original 1984 chart indicates 20g PROOF 1080 BAR = 15,664 psi.
The number was revised to 1040 BAR 11-2021.)

In closing ;) this is important.
This illustration was part of the Birmingham Proof House Trial Report of 1891
https://docs.google.com/document/d/1...hfDxmrNAU/edit

https://photos.smugmug.com/Barrel-Ev...ngth%202-M.jpg

Three 30” rough bored 12g barrels of 39 different materials (117 total) were received, and the barrels turned and bored to standard dimensions, without cutting the chamber; each barrel was fitted with a breech plug and touch hole and loading was done from the muzzle.
The wall thicknesses do NOT represent standard double gun barrel dimensions.
Note there is no chamber cut nor choke.


For the record, I've found no engineering drawings nor specifications for wall thickness for any U.S. maker. Fortunately now we have LOTS of numbers of presumed unmolested barrels with which to make rational recommendations.

Drew my understanding of Britsh proof is that a 12 ga. exceeding nominal size (.729) by more than 010" has to be reproofed before the gun can be sold in England and is marked 12/1 on the barrel flats after successful reproof. Based on that understanding Gibbs would sell a gun with a bore of .735 and is still in proof by .004. Ed Good there would be many Parkers left in dealers gun racks if .030 minimum was the accepted number.

That is correct Craig pre-1954.
1954-1989 the bore would be marked in inches, but I have seen some also marked with the 12/1.

Drew, 12/1 can mean reproof, the gun was originally built with barrels which exceeded .739 dia., or that the gun had been rebarreled which is common in England.

Correct, 12/1 = .740" - .750"; if rebarreled prior to 1954
The flats would carry the Crown over R if re-proved.

Interesting collection of proof marks. Originally proved 35-36 by 'P' on the upside-down date code, with '12' and '13/1' (.719" - .728") bore
Then reproved 1959 and marked .729"

https://photos.smugmug.com/Proof-Mar...20Asbury-L.jpg

J. Blanch & Sons original London Proof 1887-1896 with bore 12 (.729-.739").
The time bomb damascus ;) barrels Birmingham re-proved (tiny Crowned R) 1999 (BD) with bore 18.7 mm = .736" which could have been the original bore

https://photos.smugmug.com/Proof-Mar...D%3D1999-M.jpg