Proof is the process of testing the safety of a gun barrel. It is still done as it has been since the beginning of gunmaking: by firing the gun with an over-pressure charge of powder. If the gun survives, it will be safe to shoot. If it doesn’t, it wouldn’t. Although modern non-destructive inspection techniques enable the visualisation of the internal structure of metal structures to a high degree of accuracy, there is still no better way of determining which cracks and imperfections in the structure might prove dangerous than this traditional practical test.
Under the Gun Barrel Proof Act (amended 1978), proof is required for all guns sold in the United Kingdom, and for guns that have undergone structural modifications to their barrels – for example by re-sleeving worn shotgun barrels or adapting a rifle barrel to accept a sound moderator or muzzle attachment. Imported guns also need to be proofed, although that can be done at the proof house in the country of origin, provided it meets the standards laid down by the Commission Internationale Permanente pour l’Epreuve des Armes à Feu Portatives (CIP). At the London Proof House, proof is carried out in three steps:
Physical examination. The barrel and action are inspected to ensure that there are no visible signs of weakness or undue wear. The barrels are also measured and gauged to ensure that they conform to precise dimensions and tolerances.
Proof firing. Guns that pass examination are taken to an enclosed firing room, where they are secured in a holding device, which is aimed into a “snail” bullet catcher, a box containing a lubricated and hardened steel spiral that can safely dissipate the momentum of a bullet. The snails used at the London Proof House are safe for bullets upwards of .50 calibre. The gun is then loaded with a proof cartridge that, depending on the type of gun, will produce 25% to 50% more than the theoretical maximum service pressure. the firing chamber is closed, and the gun is fired remotely
Proof marks. After re-inspection and gauging, guns that pass the test, and the vast majority do, are then marked with the London proof mark. Typically, this consists of the initials GP beneath a crown, although there are special marks for unusual tests and circumstances.
In addition to proofing a steady stream of guns on their own premises, the Proof Master and proof personnel also travel to manufacturers' sites to perform proof on guns which would be difficult to bring to London, for example the armaments of the Apache helicopters used by British services.
The London Proof House has several duties in addition to proof. It is responsible for inspecting and marking firearms that have been de-activated – that is, altered in such a way as to render them permanently incapable of firing and thus completely safe for collectors The Proof House and the Proof Master consult with the government on firearms regulation. The Proof Master and colleagues also work with the CIP to set international standards for proof, and they investigate serious failures of guns to try to improve gun safety.
By far the most common gun failure involves confusion between different sizes of shotgun shells. If inserted into a 12-gauge gun, a 20-gauge cartridge will lodge against the narrowing of the barrel at chamber forcing cone. It will be invisible there, and it will leave enough room to insert a 12-gauge cartridge on top of it. If the 12-gauge cartridge is then fired, the gun will likely blow out the side of the barrel just above the forcing cone – which is dangerously near the fore-end, where the gun is held. Due to the number of such accidents, the Proof House has become involved in a campaign to urge shooters to be particularly careful about cartridge hygiene, keeping the various sizes absolutely separate.
Running of the Proof House is overseen by the Proof House Committee, which is appointed by the Court of the Gunmakers’ Company. The Proof Master, who is appointed by the Proof House Committee, is responsible for the running and operations of the Proof House. The present Proof Master is Richard Mabbitt.