doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Chamber pressure standards at London Proof House

Many thanks to all for your responses. However, the responses now bring another question to mind: What is the purpose of the "superior" ("SUP") proof? Pigeon guns? Waterfowling guns? Thank you as always.

Terry, yes they will still re-proof for black powder guns. Don't ask me what pressure system they mark as I have had barrels back that I think are just marked Black Powder Proof and the chamber length in centimetres and just prior to that I had one marked with a service load in metric that equated to 3 3/4 Dram of powder and 1 1/4 ounce of shot in a 2 1/2" 12 bore. As mentioned, we could do with an up to date book in order to be able to interpret marks.

Dougles, I think; and Smallbore may correct me on this, that the Standard proof pressure is for standard length chambers of either 2 1/2" or 2 3/4" for which they use the same proof load, and the Superior Proof is for 3" and longer chambers using a heavier proof load. That's how I understand it at the moment anyway. Lagopus.....

Lagopus,

Thanks very much.

Some 2 3/4 inch cartridges are 'magnum' loads, as are 3" shells. Superior proof is for these loads, which exceed normal pressures.

From a practical standpoint, for us Yanks shooting Brit guns: If you're going to use American factory shells in your Brit gun (other than from the specialty outfits, like RST, that load short/low pressure shells), that gun needs to have passed superior proof (previously marked 1200 bars).

I totally disagree. If the gun is proofed for 70mm standard nitro proof, you should be perfectly safe shooting 2 3/4" 'normal' loads through your guns (the likes of Winchester, which we import and use for example).

I would be extremely wary, have the gun expertly checked over by a competent gunsmith, listen and learn.
It should be safe to use commercially loaded cartridges of the correct length for the chambers, but be exceptionally wary of a recommended 'hot' homeload. Certainly never deviate from the recommended homeload recipe by being 'smart' and tinkering to 'improve' the load. With vintage guns of an uncertain pedigree or condition it is so easy to be another statistic in the A&E record books.

Thank you again for all your posts. Actually, the gun subject of this inquiry will be a new build, but built to a light weight (6 lbs. 5 oz. for 12 gauge sidelock with 30" barrels and 15 1/4" l.o.p.). Just wanted to be sure that "Standard" proof wouldn't result in a gun only suitable for light loads.

Small Bore, straight from the Birmingham Proofhouse via email I received in 2001, when the standard proof for 12 bores was 850 bars (which I do not believe has changed, even though such guns are now marked "STD" rather than "850 bars"): "The reference in the article to the CIP service pressure of 650 bar and proof pressure of 850 bar is a reference to crushers values; the equivalent transducer values that should be used for comparison with SAAMI transducer values are 740 bar service and 960 bar proof, i.e. 10,730 psi and 13,920 psi."

The current service pressure standard for SAAMI-proofed 12 bores, either 2 3/4" or 3", is 11,500 psi. Proof standard is approximately 18,000 psi. Thus, SAAMI service pressure exceeds CIP standard service pressure by almost 800 psi. While a difference that small is highly unlikely to result in a catastrophic failure, if a CIP standard proof gun is fed a steady diet of "normal" American loads, wear might well result at a faster than normal rate. I would also point out that "normal" American 2 3/4" loads would include shot charges as heavy as 1 1/4 oz with a velocity as high as 1500 feet per second. Although most target loads are more reasonable in terms of both shot charge and velocity, the problem with stating that one is perfectly safe shooting "2 3/4" normal loads" through one's guns is that we keep pushing the boundaries of "normal"--which really only means anything that's not marked "magnum".

I guess the idea of 'normal' is the one to define here!