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The CIP site states the numbers are transducer measured BAR simply converted to psi

The Commission Internationale Permanente pour l'Epreuve des Armes � Feu Portatives
Infro here
http://www.cip-bobp.org/homologation/en/tdcc_public?page=1&cartridge_type_id=7
12g 65mm and 70mm �standard proof� lead or steel (limited to no larger than 3.25 mm and max. fps 1,300)
SERVICE pressure 740 BAR = 10,733 psi; PROOF 930 BAR = 13,489 psi
Both 65 and 70 mm 16g standard is SERVICE 780 BAR or 11,313 psi; PROOF 980 BAR or 14,214 psi.
Both 65 and 70 mm 20g standard is SERVICE 830 BAR or 12,038 psi; PROOF 1040 BAR or 15,084 psi.

So what is marked on the barrels is proof and not service if I understand this correctly. So my Mckay is marked 900 bar so that is the proof and not the service load? So again I guess I am in that gray zone... TM maybe too hot for my gun

http://www.cip-bobp.org/homologation/en/tdcc_public?page=1&cartridge_type_id=7

12g 65mm � 70mm = 2 1/2� � 2 3/4� �standard proof� lead or steel (limited to no larger than 3.25 mm and max. fps 1,300) transducer psi
Service pressure 740 BAR = 10,733 psi
Maximum statistical individual pressure 850 BAR = 12,328 psi
Standard proof 930 BAR = 13,489 psi

12g 76 mm = 3� lead �High performance/superior proof� transducer pressure
Service 1050 BAR = 15,229 psi
Maximum statistical individual pressure 1200 BAR = 17,405 psi
Magnum proof 1320 BAR = 19,145 psi

Rev Drew/Bro Larry,

I have a WC Scott, 28ga. made in 1978, Birmingham address. It has factory 2-3/4" chambers and it shows 3-1/4 tons. The gun was made for Orvis as it says "Orvis Best" on the trigger plate. It still makes me scratch my head why they would show 3-1/4 tons, made for the American market with 2-3/4 chambers.

One other interesting gun is my Parker VH 20ga with English proofs.

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

BUT the 1954 numbers were also measured by lead crusher units (LUP) rather than transduced psi, which would be about 10% psi higher.

However, the CIP standard proof pressure of 850 BAR (kg per sq. cm.) = 10,730 psi service pressure.

This implies that the CIP conversion for the 1954 TONS measurement is different from the 1989 BAR measurement.

Those post-war Webley & Scott guns have also been something of a quandary for me. By their markings--and the tons are service pressure rather than proof pressure, as Drew indicated--they fall a little short of SAAMI service pressure standards even when bumped up from LUP to psi, yet were made for export to the USA. Across the board, they're pretty light guns. I wouldn't hesitate to shoot standard American target loads in them, preferably with velocities on the modest side, but I wouldn't want to shoot the Winchester 1 oz 28's in a 728, nor a 1500 fps 1 1/4 oz field load in a Model 700 12ga. I owned a very nice 720--basically a 702 20ga made for the American market. 3" chambers. That gun weighed right around 5 3/4. One ounce loads were none too comfortable. I can't imagine magnums. But I guess the Brits realized we Yanks had a case of magnumitis back then, and were especially enamored of the 3" 20.

RevDoc Drew,
Bro Larry gracefully corrected me on my bar numbers earlier.
After seeing your last post, I was making the same crusher mistake for tons.

Bro Larry,
I never have figured out what model my 28ga is. The action is much more rounded than the 720's and 728's I have seen. Also there is no engraving but it has knock out wood.

Thanks,
Bill

Bill, you may have something other than a 728. But since your gun has the "tons" proofmarks, that certainly puts it in the 700 series era. If you're sure about the date of production, it could be that W&S produced a special series of guns for Orvis. Have you tried checking with Orvis? Does it have a serial number in the 728 range? All of those are 728xxx.

Brother Larry

From the Doubleguns list, 146xxx shows 1978. I haven't contacted Orvis. My thought was the guys on here are probably more knowledgeable.

DT's
25" barrels
Churchill rib
Marked WC Scott Birmingham
"Orvis Best" on trigger plate
3-1/4 tons on flats
2-3/4 on flats
Cased

Not sure yet what I have but its a real keeper.