In this video the gunsmith Jack Rowe seems to indicate that gauge tolerance works in both directions. 12 gauge could be anywhere between 0.710 and 0.751.

In part one of the video series he states that 0.008 (0.010 for 1954 requirements) is the most a bore can be enlarged and stay in proof. If it is bored out more than 0.008 above its proof dimension then it is out of proof and must be reproofed.

https://www.youtube.com/watch?v=PtvfSHr5TR4&list=PLB89EB25199C06E73&index=2

Stephen