The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

I have seen a few discussions about reproofing barrels after work has been done which appear to only apply to rifles, and usually more often rimfire, cutting down, re-threading, crowning etc and the threat of breaching some section of the firearms act.
However, I've never seen anything about re-proofing modified S1 shotgun barrels eg re-threading for choke or chopping and fitting muzzle brakes. So does this not apply to shotguns in any way?

Same rules apply to shotguns.

Not the proofing part, but pay special attention to S6 of the Firearms Act 1988!!

https://www.legislation.gov.uk/ukpga/1988/45/section/6

It creates stupid oddities in the firearms regs where a shotgun bought in a certain configuration is legal, a gun you add a different (shorter) barrel would be legal but if you shortened the barrel you already have you could be committing an offence

The proof act states that any barrel that "be unduly reduced in Substance or Strength " is deemed unproved (s111).

It has been argued that a barrel shortened isn't reduced in substance or strength within the meaning of the GBPA.
Not sure how this works with shotgun barrels but I would guess if the bore has been reamed out the proof marks might not represent the proof status of the barrels so it's back to the proof house for them. However if you can argue the substance and strength hasn't been unduly reduced and the marks still represent the status of the barrels then it might be OK. It would be the smith who does the work and hands it back to you that would be the one that needs to be convinced as he'd be liable I guess.

As with most things regarding proof it is not very scientific or ultimately safe.

I stand to be corrected, but is the reason for the proof house now stamping at the muzzle end of the barrel as well done to negate the grey area of “substance or strength” in that any work would remove the proof mark, which in itself is an offence if the firearm is then not sent back for proof?

I believe that as a result of them having no answer to the question of shortening doesn't make it weaker so therefore not reduced in substance or strength that they did just this.