Lottery odds. No two British "fine" guns were made anything but one at a time. What if your barrels are struck .010 more on the right side and .010 less on the left so that one side against the breech face the right is too small and the right too large. Are you going to strike the breech down on the right and the barrels on the left? Nope.

Before I ever tried to fit a set of barrels I spent a lot of time measuring everything you could think of before I started. To me fitting a set of barrels, which were too large or too small for the gun would have been a waste of time and a set of barrels. But you never know. The lottery does pay off for someone.