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Question re: a nitro proof mark
#496694 12/04/17 12:21 PM
Joined: Jan 2002
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western Montana
Robt. Harris Offline OP
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Robt. Harris
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Hi Fellas,

Ive a shooting friend who recently purchased a very nice condition Cogswell & Harrison false side-plated 12 ga. boxlock c. 1908-1909 to serve as a shooter. It is still current in proof with a 12 and nominal bore measurements of .730 and .731 respectively. It has all of its original nitro-proof stamps (1904 Rules) that includes: NITRO PROOF 1-1/8 OZ.

I was asked to fly-speck the barrels prior to his purchase, as I have the various gauges, and was surprised to find the chambers at a measured 2-3/4. The lead-ins to the forcing cones begin right at that measurement and are of the short (approx. ) length that one often sees from guns of that era. In short, everything about the gun and bores looks straight-up original right down to the level of polish within the cones themselves. The minimum wall thickness over both forcing cone lead-ins is .115, the barrels overall have MWT of .034, and weigh 3 lb. 4 oz. with their 30 tubes..which makes for a pretty stout set.

Finding the longer chamber length still has me thinking that I should be seeing the heavier nitro proof stamp of: NITRO PROOF 1-1/4 OZ. ..and Im looking for some guidance. Ive gone all through Wirnsbergers book on proof marks and am still left with the question as to whether the London House would ever put the lesser proof stamp for a 2-1/2 inch gun on one sporting 2-3/4 chambers? My suspicion is that they would not, but wouldnt mind being corrected by those more knowledgable. The owner is completely content with the longer chambers, and would use the same lower pressure loads that he now uses in his other vintage gun.

Any thoughts on this nitro-proof stamping ???

Rob Harris
Re: Question re: a nitro proof mark
Robt. Harris #496697 12/04/17 12:50 PM
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Red State
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Originally Posted By: Robt. Harris
Hi Fellas,

Ive a shooting friend who recently purchased a very nice condition Cogswell & Harrison false side-plated 12 ga. boxlock c. 1908-1909 to serve as a shooter. It is still current in proof with a 12 and nominal bore measurements of .730 and .731 respectively. It has all of its original nitro-proof stamps (1904 Rules) that includes: NITRO PROOF 1-1/8 OZ.

I was asked to fly-speck the barrels prior to his purchase, as I have the various gauges, and was surprised to find the chambers at a measured 2-3/4. The lead-ins to the forcing cones begin right at that measurement and are of the short (approx. ) length that one often sees from guns of that era. In short, everything about the gun and bores looks straight-up original right down to the level of polish within the cones themselves. The minimum wall thickness over both forcing cone lead-ins is .115, the barrels overall have MWT of .034, and weigh 3 lb. 4 oz. with their 30 tubes..which makes for a pretty stout set.

Finding the longer chamber length still has me thinking that I should be seeing the heavier nitro proof stamp of: NITRO PROOF 1-1/4 OZ. ..and Im looking for some guidance. Ive gone all through Wirnsbergers book on proof marks and am still left with the question as to whether the London House would ever put the lesser proof stamp for a 2-1/2 inch gun on one sporting 2-3/4 chambers? My suspicion is that they would not, but wouldnt mind being corrected by those more knowledgable. The owner is completely content with the longer chambers, and would use the same lower pressure loads that he now uses in his other vintage gun.

Any thoughts on this nitro-proof stamping ???

Rob Harris


I doubt they would ever understate the chamber lengths. If they did it once and it was a known fact what would stop someone from cleaning up a pit or two by going from 2 /1/2-2 3/4? Then claiming the barrels had always been that long to begin with. I'd measure the wall thickness where they were reamed out just to make sure it's not too thin.
Re: Question re: a nitro proof mark
Robt. Harris #496703 12/04/17 01:16 PM
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Your evidence points to a lengthened chamber, that has been that way for a long time. It is most definitely out of proof. That means you can't sell it in England. If it is beat up and a little loose, somebody probably shot whatever would go in the chambers in it. If it's in good shape, user took care of it and didn't use cases of 1 1/4 super-X through it.
Re: Question re: a nitro proof mark
Robt. Harris #496717 12/04/17 03:23 PM
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Suffolk, UK
Toby Barclay Offline
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Almost certainly lengthened since proof. Never come across a genuine 2 3/4" marked for 1 1/8 oz. But, as they say, there is always a first time! And I have seen plenty of Proof House mistakes over the years.
Nowadays the pressure for 65mm & 70mm proof is the same but I am pretty sure that wasn't the case back then.
Re: Question re: a nitro proof mark
Robt. Harris #496731 12/04/17 05:35 PM
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AZ but dreaming of KS
Drew Hause Offline
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British Service and Proof pre-1925
12g 2 1/2 and 2 5/8 chambers (bore .710-.740) for a maximum service load of 3 1/4 Dram Eq. with 1 1/4 oz. shot. (1220 fps)
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dq
Definitive Proof 6 1/2 Drams Proof-House Black Powder with 1 2/3 oz. No. 6 shot = 10,100 psi + 10 - 14%
Supplementary Nitro Proof with 4 1/2 Drams of C&H No. 2 T.S. powder and 1 2/3 oz. shot = 16,400 psi + 10-14%

That certainly compares with modern 12g service and proof loads

Post- 1925 the 2 1/2 & 2 5/8 12g max. service load was reduced to 3 Dr. Eq. with 1 1/8 oz. shot with a mean pressure of 3 1/4 tons by LUP = 9,682 psi by Burrards conversion.

Wall thickness of .115" at the end of the chamber and at the forcing cones by all recommendations is adequate.

https://sites.google.com/a/damascusknowledge.com/www/home
Re: Question re: a nitro proof mark
Toby Barclay #496778 12/04/17 11:55 PM
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western Montana
Robt. Harris Offline OP
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I want to 'Thank' you fellas for confirming what I suspected..... your explanations furthering my overall understanding. As I mentioned, the owner is content with the lengthened chambers as he really likes the gun's stock dimensions, overall condition, etc., so nothing will come of this other than his learning to get the vetting done prior to conclusion of a purchase. Again, my 'Thanks' to you.....

All Best,

Rob Harris

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