The Proof House has statutory duties to regulate the safety of firearms in the United Kingdom. Its three major activities are: proof, safety and de-activation.

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Proof is the process of testing the safety of a gun barrel. It is still done as it has been since the Company was established in 1637: by firing the gun with an over-pressure charge of powder. If the gun survives undamaged, it will be safe to shoot. If it doesn’t, it is deemed unfit for sale – and kept from the hands of the public. Although modern non-destructive inspection techniques enable the examination of the internal structure of metal to a high degree of accuracy, there is still no better way of determining which weaknesses and imperfections might prove dangerous to the user.

The Gun Barrel Proof Acts specify the rules under which proof is carried out. They require proof for all guns sold in the United Kingdom, and for guns that have undergone structural modifications to pressure-bearing parts – for example by sleeving worn shotgun barrels, or adapting a rifle barrel to accept a sound moderator or muzzle attachment. Imported guns also need to be proved, although that can be done at the proof house in the country of origin, provided that country is a member of 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.

A Proof Assistant inspects action and barrel prior to proofing. He is looking to ensure that the action fits cleanly to the barrels. He will also examine the interior of the barrel to ensure that there are no deformities or pitting. Note that these new barrels are “in the white”. Once it has passed proof, it will be returned to the factory for bluing and polishing.
A Proof Assistant inspects action and barrels prior to proving. He is looking to ensure that the action fits cleanly to the barrels. He will also examine the interior of the barrels to ensure that there are no deformities or pitting. Note that these new barrels are “in the white”. Once they have passed proof, they will be returned to the factory for bluing and polishing.

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 up to and including .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.

The firing jig used for proof rounds. Shot is caught in the bullet catcher to left, and the jig will roll back to absorb recoil. The trigger is pulled with a loop of string pulled through the door. Note that the stock and other fragile parts have been removed to minimise the chance of damage.
The firing jig used for proof rounds. Shot is caught in the “snail” bullet catcher to left, and the jig will roll back to absorb recoil. The trigger is pulled with a loop of string pulled through the door. Note that the stock and other fragile parts have been removed to minimise the chance of damage.

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.

The end of a successful proof. The initials “GP” (for Gunmakers Proof), topped with a crown, as specified in the Gunmakers’ Charter of 1637 are stamped on to a barrel to show that it has passed safety tests.
The end of a successful proof. The initials ‘GP’ (for Gunmakers Proof), surmounted by a crown, as specified in the Gunmakers’ Charter of 1637 are stamped on to a barrel to show that it has passed safety tests.

In addition to proving a steady stream of guns at the Proof House, 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 the Regular Army.

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 Company consults with the government on related firearms regulation. The Proof Master and colleagues also work with the CIP to set international standards for proof, and they investigate serious pressure-related failures of guns to try to improve gun safety.

By far the most common gun failure arises from shooters confusing the different sizes of shotgun shells. If inserted into a 12-gauge gun, a 20-gauge cartridge will lodge against the narrowing of the barrel in the 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 frequency 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.

The Proof House Committee, which is appointed by the Court of the Gunmakers’ Company, is responsible for regulating and executing the proving of small arms in accordance with the Company’s Charter and the Gun Barrel Proof Acts. The Proof Master, who is appointed by the Proof House Committee, is responsible for the day to day running and operations of the Proof House. The present Proof Master is Mr Richard Mabbitt.