With reference to the final paragraph of the linked note from the two Proof Houses which apparently was intended to clarify the status of a ‘muzzle attachment’
‘Under current Proof Legislation, the proof status of any muzzle attachment changes as soon
as it is fitted onto the barrel of a small arm. Thereafter it becomes part of the barrel of a small
arm and requires to be proof tested and marked accordingly prior to attempting to export, sell,
exchange, pawn or pledge the attachment together with the small arm to which it has been
fitted (see Section 4 of the 1868 Act and the 2006 Rules of Proof).’
It’s concerning that such two bodies use such woolly & imprecise language. If their clarification is taken at face value then apparently the factory supplied sight base extension tube clamped on the muzzles of some .22 LR target rifles are required to be proofed as would presumably the RPA & Fultons height adjustable foresights which were clamped around the muzzles of my Fullbore TR rifles. Nowhere in the Proof House note does it say that the accessory has to withstand pressure - so if I was to tie a piece of string around the muzzle of a rifle would that need to be proofed? It would appear so if the advice of the Proof Houses is to be given any respect.
Sadly the paragraph from the Proof House guidance is just an example of badly written definitions/clarifications - the same applies to parts of the current firearm act covering moderators/attachments.