Barrel wall thickness

[Dean Romig]

Originally Posted by Bill Murphy

Deleted

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





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[Drew Hause]

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



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.

[Drew Hause]

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



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.)

[Drew Hause]

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



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.