Where does one test gunbarrels in a big city? In London, one tests them at the Proof House of the Gunmakers Company, explains the Caroline’s Miscellany blog:
On Commercial Road, E1 is a rather curious-looking little building, out of scale with the structures surrounding it… The current building of 1757 replaces one damaged by repeated explosions.
By law, all gun barrels have to be tested; they also have to be retested after undergoing structural modifications. This is done in three stages: first, there is a physical examination which includes measuring and gauging the barrels. Then the gun is fired, using a special cartridge to generate more pressure than should be experienced in normal use. Given the as-yet unproven state of the weapon, it’s reassuring to read that it is fired remotely in a closed chamber! The bullet is caught in a ‘snail’, a hardened steel spiral. Finally, provided that it has passed these tests, the gun receives a proof mark.