Could I make a sound moderator for a semi- automatic shotgun?
It would require a special, ported choke tube which would extend the muzzle by about 40mm, and a baffle chamber which would surround the choke tube and the forward end of the barrel, giving clearance to the rib.
Advantages would be that the gun would not have to be re-proofed, and there would be no need to buy a spare barrel.
Shotgun technical advice
On the face of things, this sounds like a neat idea. But the gun would have to be re-proofed with the moderator in place.
The proof regulations are quite clear on this point, and I quote from a very useful download from the Birmingham Proof House website.
It is a very concise guide to general barrel modifications as well as ideas such as yours: “The Trade and the public are advised that the barrels of small arms fitted with sound moderators, screw in choke tubes, muzzle brakes, etc. require to be proof tested. In addition any barrel of any small arm previously proved but subsequently converted by a manufacturing process, resulting in a reduction in substance or strength, to accept the fitting of sound moderators, screw in choke tubes, muzzle brakes, etc., will require the arms to be reproved.”
“This advice also applies to any alteration to the internal dimensions of the barrel affecting bore size or chamber lengths in excess of permitted tolerances. Muzzle attachments through which any part of the charge would be exploded or discharged form part of the barrel and require to be proof tested and marked accordingly.”
To you, the last sentence of this advice is the most relevant.
Also, although I am not an expert on the subject, I don’t think you would get enough gas escape through 40mm of tube to achieve very much in the way of sound moderation.
It’s an interesting idea, but I don’t think it would work. And if it did, it would throw the gun very badly out of balance.