Switch barrel guns were popular in the same era as takedown guns. In the days when hunters travelled by train, a compact gun case was highly desireable as it would easily fit into the overhead rack or beside the owner. Obviously, if a rifle or shotgun can be made as a takedown, it is not much of a stretch to make another barrel in any caliber that has the same breech face.
But, the OP is not interested in building such a rifle, only in properly installing a barrel on a Model 1903. Unclenick's advice is good, but setting the barrel back is not really an option if the configuration of the original rifle is to be retained. If the rifle is a Model 1903, the rear sight base has to be removed and replaced, plus the front sight has to be kept aligned. Then the bayonet will not fit properly since the barrel will be too short. The OP's best bet may be to either roll the shoulder as Unclenick suggests, or try to find another barrel that will draw up and headspace properly.
Jim
But, the OP is not interested in building such a rifle, only in properly installing a barrel on a Model 1903. Unclenick's advice is good, but setting the barrel back is not really an option if the configuration of the original rifle is to be retained. If the rifle is a Model 1903, the rear sight base has to be removed and replaced, plus the front sight has to be kept aligned. Then the bayonet will not fit properly since the barrel will be too short. The OP's best bet may be to either roll the shoulder as Unclenick suggests, or try to find another barrel that will draw up and headspace properly.
Jim