doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com British Nitro 12 Bore Service Pressure?

Hi!
I am purchasing a mint condition British side by side shotgun made sometime between 1904-1925. It has the nitro proof marked, 1 & � (oz of shot), 12 bore and 12 LC marked.
It has 3 inch chambers.
My question is what is the service pressure that this Waterfowling shotgun was originally proofed?? I have found the below information which only partly answers the question.

British proof markings: The "tons" mark (which is service pressure) was not used until 1954. Between 1925-54, the chamber length was marked.
1904-1925 guns would not have the chamber length, but only the shot charge and a nitro proof.

Per the 1954 Rules of Proof, here are the equivalent service pressure values:
3 tons--8,938 psi
3 1/4 tons--9,682 psi
3 1/2 tons--10,427 psi
4 tons--11,917 psi
These numbers SHOULD apply to guns proved under the 1925 rules, which were stamped only with chamber length and shot charge. Thus, a 2 1/2" chambered gun, 1925 rules of proof, would be equivalent to 3 tons; a 2 3/4" chambered gun to 3 1/4 tons.

A service pressure of 3 tons per sq. in. = 8,938 psi service pressure (SAAMI)

It is proofed for the old 3 inch magnum rounds and is not considered safe for the modern 3 inch cartridge. It is generally considered o.k. for the new 2 3/4 inch semi magnum rounds. Easy to reload for using 3 inch cases and anyway it was only really made to handle 1 1/2 ounce of lead and heavier loads may kick a bit. Lyalvale Express make a neat 1 1/2 ounce load that only generated 2 3/4 inch pressures. I understand that brand is available in the U.S. I use them through choice and they perform well. Lagopus.....

I believe I know what gun you are buying, and I'm glad you are, it will prevent me from spending the money!

I have a later gun (approx 1940's) 3" chambered, 1 1/2" oz load marked. However, it is also marked with the 3 1/2 ton designation. While your earlier gun does not carry the proof marked in "tons" the 1 1/2 oz marking should put it in that same category. The advice to keep pressures within 2 3/4" limitations is sound. Keep your loads under 10,000 psi.

And please post some pics when you get it!

Flintfan, your 40's gun must have been reproofed after 1954. The 3 1/2 ton mark wasn't used before then, and the 1 1/2 oz mark goes back to the 1925-54 era proofmarks.

Yes, thank you for the correction. It was made post 1954. I had a brain lapse there.