6 BORE has internal diameter of 0.919" (Burrard)
23.4mm = 0.921" when measured at re-proof, the owner presumably decided on having it proofed for Nitro powder.....ie the gun is still a 6 bore - as constructed.
At re-proof, 4 in diamond was struck, to show that it is chambered for the 4 gauge cartridge. So in answer to your question it is a true Six bore although it is loosely termed a 4 bore- because that is the dimension of the cartridge for which the gun was built.
You will note that the level of proof is 800 bar which is well below that of the modern 3 1/2" 10 G. Sadly the other Proof House caused not a few tears for those 4 bore owners who sent their gun for nitro -and which subsequently failed (sometimes irreparably) when subjected to test pressures calculated from CIP rules .....!! Let this be a warning to any owner of these old guns.
