Fred:  Your question got me measuring some Krag and 03 & 03-A3 Springfield barrels.  My 03-A3 steps-up from .620" (in front of sight band) to .649" (behind the front sight band).  My RIA 1903 steps-up from .621" to .647".  I never realized that 1903 & 03-A3 barrels change diameter so abruptly.  This must create a shoulder the front-sight band rests against.
  '98 & '99 Krag Carbine barrels I measured were .655" in diameter at the front-sight base.  (The Krag barrels have a smooth taper and no abrupt diameter change).
  An 1898 rifle I have that's barrel was cut to about 22&1/4" measures .650" in front of a (unknown maker) banded sporter front-sight.  Another 22&1/4" shortened rifle barrel, with a Marble's banded front sight, steps-up from .627" to .660".  I suspect gunsmiths and Springfield turned Krag barrels in a Lathe and machined a step at the muzzle to accommodate the smaller inside diameter of the '03 sight.  A good machinist could do this without taking the barrel off the receiver.  Another possibility is to bore the '03 or '03-A3 sight base to a larger inside diameter.  (I prefer the look and simplicity of the '03-A3 front sight over the 1903).