I had always been under the impression that the change from a gauge mark, as 12, 12/1 etc was to get away from this "Step" method & that when the decimal use started they were actual bore size, not somewhere twixt & tween so to speak.
Thats also my understanding.
The gun I recently had re-proofed follows the decimal marking to the tenth millimeter. The Rules of Proof 2006 also state bore shall be measured at 23cm using a tool capable of detecting 1/10th mm variation. So if it's a go-no go or a micrometer they're measuring the "start " point to a limited range.
Jeremy
