Not in Proof by U.K. standards possibly but it has not had the strength compromised but I would in no way put 3" cartridges through it as the pressure would be an issue then. A 16 bore can be between .661 and .681 and still be marked as a 16 bore. The Bore is on the tighter side of the 16 bore barrel. I'd consider having it put back as a 16 bore and using it as such. Lagopus.....