A couple of quotes from Burrard;
Vol I page 17: Bore - that part of the inside of the barrel which is sealed completely by the shot sharge & wads as they travel along the barrel on discharge > The size, or Gauge, of a gun depends on the diameter of the bore, but this is not measured in decimals of an inch, but is denoted by the number of spherical balls of pure lead, each exactly fitting the bore, which go to the pound.
Now after this entirely correct description & terminology lets look at how he applies it;
Vol III, page 313: The Gauge division 12/1 includes any bore of diameter .750" to .740" inclusive; and the gauge division 12 any bore of diameter .729" to .739". If the diameter is .751" the gun is not a 12-bore but it's gauge size is 11, and the gun is proved as an 11-bore.
