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:
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.