The different bore size--as long as it isn't TOO different--doesn't make much difference as long as the chamber size is standard for the particular gauge in question. Barrels a little overbored or underbored? No big deal. But in the case of the Brits, they wanted to know and indicate the exact bore size on the barrel flats via the fractional (/1) marks. That's because, by their standards, a gun is out of proof if it's overbored enough to move it into the next gauge category. For example, if a barrel marked 16/1 (.669 standard) measures .677 or more, then it's a 15 gauge and would legally require reproof. The thinking is that if it's overbored too much, then the barrel walls may no longer be thick enough to pass proof. Someone got overly ambitious with a hone.
