doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Thomas Bland BLE proof marks

It seems strange to have 1 barrel .729 and the other .719 but I have a theory, based on having owned several Britt boxlock waterfowlers. At the time some of these guns were made shells with felt wads were used and the gunmaker wanted to insure combustion and perhaps increase performance by having tighter constriction in the barrel.

Given the manufacture date of - I think - 1963, do you think that's still a possibility?

OK. Found several sources stating "A significant change in 1954 was the requirement to stamp the actual measured bore diameter on the barrel flats, not just the nominal gauge number."

Geoff: I agree the Birmingham date code is 'O' for 1963

Mike: it's complicated
Pressures can be converted to modern piezo transducer numbers using Burrard’s formula if originally expressed as (Long) Tons/Sq. Inch by LUP (Lead [Crusher] Units of Pressure), or by adding 10-14% if expressed as PSI by LUP.
Burrard’s estimated Long Tons/ Sq. Inch Lead Crusher Pressure (Cp) conversion to PSI (pound-force per square inch): (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI.

“Highest Mean Service Pressure” equivalent PSI transducer values as converted from LUP by Burrard’s formula:
2 tons/sq. inch (NOT 2 X 2240) = 5,600 psi (pounds/sq. inch) = 386 BAR
2 1/2 tons = 7,280 psi = 502 BAR
2 3/4 tons = 8,120 psi = 560 BAR
3 tons = 8,960 psi = 618 BAR
3 1/4 tons = 9,800 psi = 676 BAR
3 1/2 tons = 10,640 psi = 734 BAR
3 3/4 tons = 11,480 psi = 792 BAR (The SAAMI maximum for 2 3/4” 12g is 11,500 psi)
4 tons = 12,320 psi = 850 BAR