Note that under the "Old" system as Drew has stated the marks are not actual diameters but simply the largest plug gage in a series which would enter the bore to the required depth of 9".
To give an example of "Possibilities", consider two barrels have gone through proof. One had a bore diameter of .7295 & the other .7395.Both would accept the 12 gauge plug but neither the 12/1 plug so both would be marked 12. The smaller barrel could have the bore enlarged by .010" & remain in proof as it would still not accept the 12/1 plug. If on the other hand the larger one were enlarged by only .001" it would then accept the 12/1 plug & be "Out of Proof". This situation "Eventually", after many many years, led to the marking of actual bore diameter. From that point on the gun was considered In Proof until it was enlarged by .2 mm (.008"), a much better system. Barrel wall thickness is of course what determines whether a chamber can be lengthened or not. Many guns have had their chambers safely lengthened, including many British guns, which do have to be re-proofed in order to sell one of them. Even many guns which had original 2 3/4" chambers should not be used with modern SAAMI spec shells, even with the longer chambers they were not proofed for that pressure level. The load put in that shell is of far greater importance than the actual length of the hull.
"NO" shell should be used however which has a "Loaded"length long enough that the crimp enters the forcing cone as this can delay the opening of the crimp & may raise the max chamber pressure to a dangerous level.
Also guns with a "Stepped" or very abrupt cone should not be used with hulls longer than the chamber as the crimp will then open into the actual bore.
