If its marked ay 18.4mm which in fact is .724409"then it goes out of proof at 18.6mm or .732228".
When boring for proof you needed to bore to to .725" effectively to accept a 18.4 mm plug .( you can not put a 18.4 mm plug in a 18.4mm hole without a hammer )
Proof Houses have , in my experience been at times very strict ,so if the barrel would not take an 18.5mm plug it would have been marked as 18.4 mm
In other times they have been a little lenient and marked up in this case at 18.5mm .
That said after 2010 with yet more CIP rules ( another story here ) and getting new gauges to comply with these rules they did become very strict on bore sizes and chamber depths .
To those of us who did submit guns for proof , new barrels as well as reproofs it did become a bone of contention , not to mention a headache trying to figure out why for no reason all our cutters and gauges were no longer " acceptable ".
As Drew says the Proof Houses are never wrong . Even when they are and in my personal experience they can be and have been .
