It's a matter of perspective. To the Criedmore crowd big bore probably starts at 27 caliber. To most of the American shooting fraternity 375 seems to be the cut off. In Britain my understanding is they don't consider it big bore until it is .45 or larger. To Wolfe Publishing it's 8mm or bigger if you go by their Big Bore reloading book.
Chronographs, bore scopes and pattern boards have broke a lot of hearts.
