The stamps on British guns showing the original bore size are very important factors in determining whether the gun is still in proof. If the current bore size is within ten thousandths of an inch (10 thou’) of the last size stamped, it is still in proof. The 1984 -1989 rules metricated the stampings and the closest equivalent is shown on the right. Metric stamped guns go out of proof once they are 8mm larger than last stamped.
Note that wall thickness, which is arguably more important in assessing the strength and serviceability of a gun barrel, is not a legally recognised proof issue. A gun may be legally sold with any wall thickness as long as the bore measurement, taken 9″ from the breech, is within the above mentioned tolerances. Gun buyers are advised to pay close attention to wall thickness before buying a gun, as it will determine the likely life left in the barrels and therefore, affects value to a significant degree.
12-bore nominal sizes
Calibre in inches Bore size Metric
.740 12/1 18.8
.729 12 18.5
.719 13/1 18.3
.710 13 x
16-bore nominal sizes
.669 16/1 17.0
.662 16 16.8
.655 17/1 x
.649 17 x
.637 18 16.2
20-bore nominal sizes
.626 19 15.9
.615 20 15.6
.605 21 x
.596 22 x
Chamber Lengths
The size (or length) of the chamber of a British gun was used to define the power of the cartridge used in it. For this reason, it is important to use only cartridges with the appropriate length cases in guns stamped accordingly. 2 1/2″ stamped guns are not tested for loads commonly found in 2 3/4″ cases. Additionally, increased pressure is a result of the longer cases not opening fully in the shorter chamber (torn case ends are commonly observed in such cases).
Chamber Length 1925 Stamping 1954 Stamping Service Pressure
2 1/2″ 1 1/8 oz 3 Tons 8,943 psi
2 3/4″ 1 1/4 oz 3 1/4 Tons 9,686
3″ 1 1/2 oz 3 1/2 Tons 10,427
3″ Magnum x 4 Tons 11,913
Published by Vintage Guns Ltd on (modified )
