doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com proof question concerning bore diameter

I've been wondering for a while now about the system used on British guns regarding bore diameter
please correct me if I'm wrong but were they not tested at the proof house with ball type guages then marked with a size (13 over 1 for example)
given that there is a .010" step between gauges is it not possible that a barrel stamped 13 over 1 (.719") could have actually been .725" (or bigger) but the 12 ball gauge didn't fit?
or am I well of base here? just curious

If you are wrong you are not alone.

That's my interpretaion of it also.

The bore is "plugged" with a plug gauge of a set size . If it enters then the pug of the next size up is tried . If this second plug with not enter then the bore is stamped at the lower size .
As a result of this some imported gun ,bored to metric sizes were stamped up at a lower size even if they were any effectively .001" under . As a result of this the proof houses plugged imported guns with an intermediate gauge and stamped to the nearest size up or down . British proofed guns were not treated like this so it was up to the submitter to bore as appropriate .
In current metric sizes in steps of .01mm or .004"it is less critical and gives a greater range of bore sizes .