Craig, if you are using a ball type bore gauge and calibrating it with a .700 ring I have found that when measuring a 10 bore the reading will be .003 to .005 larger than it actually is. Possibly even more. I lathe made a .775 adjusting ring to eliminate this "creeping" error. I think the error is due to the fact that as the three balls extend out in a larger bore the measuring rod that extends to the dial caliper does not change in a direct ratio to the movement. Clear as mud? Just trust me there is a significant error....
With a .700 ring I find that if a bore is only .729 there is a couple thousanths error due to the balls.
