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07-05-2023, 06:05 PM | #83 | ||||||
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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.
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07-05-2023, 06:41 PM | #84 | ||||||
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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.)
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07-05-2023, 06:52 PM | #85 | ||||||
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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.
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07-06-2023, 09:14 PM | #86 | ||||||
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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.
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07-07-2023, 07:38 AM | #87 | ||||||
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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.
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07-07-2023, 09:32 AM | #88 | ||||||
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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.
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07-07-2023, 10:10 AM | #89 | ||||||
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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" 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
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