(A) To be sold legally, every cartridge firearm must bear valid proof marks. As the UK is a signatory of the International Proof Commission (CIP), it recognises the proof marks of every other member state. For example, this means that a gun with Belgian proof marks can be sold in the UK without being re-proofed because Belgium is a CIP member. However, as the US has no federated proof system and American makers conduct their own proof tests, US guns must be proofed on arrival in the UK, because the US is not a CIP member.

A gun is said to ?go out of proof? if the internal dimensions of its barrel exceed certain defined original dimensions when it was proof tested. This can be caused by corrosion, wear, excessive cleaning with abrasive material, or the deliberate removal of metal ? for example, when a gun is rebored.

The Proof Acts require a gun to be re-proofed if it has been ?materially weakened? by repair work such as reboring or weld/brazing of the action. Equally, the screw-cutting of a barrel for a sound moderator, the installation of new screw-in chokes and fitting a muzzle-brake require a re-proof.

Guns can also be re-proofed to take different ammunition ? any gun which has had its chamber lengthened must be re-proofed.