Ok….not to split hairs….
If the wood diameter at the muzzle is 1.002, and at the middle it measures .602, the diameter across the “skinniest” part vs the muzzle is still .400 less than the muzzle.
The measurement has nothing to do with the “fatness” or the amount of wood the builder chooses to make it. In other words, as long as the same amount of wood follows the barrel contour, that.400 difference will always remain the same.
. (And yes, leaving one-thousandth of wood is impractical if not impossible.)