No it does not need proofing either cut or thread
The proof house will argue differently but then they make money on proofing!
Counsel's Opinion on the British laws of Proof relating to sound moderators and other muzzle attachments
Legal Counsel has now confirmed that:
A sound moderator is not a “part” of a “small arm” for the purposes of the Proof Acts.
There is no requirement in law for a sound moderator to be subject to proof.
The cutting of a screw thread on a barrel in order to affix a sound moderator is most unlikely to “unduly reduce it in substance or strength”, although each case will depend on its own facts.
The pronouncement made by the Proof Authority in October 2001 is misleading and wrong in law in a number of important respects.
The full text of Counsel's Opinion may be downloaded and read here:
On 31 August 2000, the London and Birmingham Proof Houses issued a circular entitled "Screw cutting of rifle muzzles and the fitting of sound moderators and other items". A further circular entitled "Conversion of Barrels and the Fitting of Muzzle Accessories" was issued by the Proof Houses in October 2001.
The contents and inferences of these circulars have been hotly disputed by the gun trade, and we consider that they are misleading to the extent that they convey the impressions:
(a) that there may be a legal requirement to submit all sound moderators for proof and,
(b) that there may be a legal requirement to submit all rifles for proof or re-proof after the cutting of an external screw thread at the muzzle.
Section 122.(3) of the Gun Barrel Proof Act 1868 ("the 1868 Act") creates an offence of selling or exchanging, or attempting to sell or exchange, a Small Arm the Barrel or Barrels of which are not duly proved or marked as proved.
However, the terms "Small Arm" and "Barrel" are separately and distinctly defined in the Act and from these definitions it is clear that there is no offence of selling, etc. an unproved "Barrel" on its own. Therefore, even if sound moderators were deemed to be "Barrels" as defined by the 1868 Act (a suggestion which is hotly disputed), there is no legal requirement to have them proved unless they are sold as part of a "Small Arm".
Similarly, with regard to the cutting of a screw thread on a rifle muzzle (or for that matter any other repair, replacement or modification of a Barrel), there can be no offence committed unless or until the entire Small Arm is subsequently offered for sale, exchange, export etc. Even if/when this were to happen, the requirement to submit a barrel for re-proof only arises if the barrel has been "unduly reduced in Substance or Strength".
As professional engineers, we reject any suggestion that a threaded rifle muzzle with a factor of safety against failure of more than four (as in the extreme case of a 1/2" UNF thread on a typical 7.62 mm rifle) is "unduly reduced in Substance or Strength", just as similar suggestions were dismissed in written evidence for the case of Regina v Beatham (see R-v-Beatham_evidence.pdf - 986 kB Adobe Acrobat file). We note that on receipt of this evidence, the Crown abandoned its case and the Defendant was duly discharged with his costs awarded from the public purse.