Bore size proof marks

The data you need to understand your bore stamps.

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Gunsmithing & Technical|June 2018

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 )

Gunsmithing & Technical|June 2018

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