The weakest link when firing a chambered cartridge is the cartridge case itself.  The U.S. Krag has a rimmed cartridge and headspaces on the cartidge rim.  The case walls are very well supported in the Krag chamber.  The action receiver has a single heat-treat which leaves the metal relatively hard and brittle.  Fortunately, if a cartridge case fails in a Krag, the cartridge rim keeps the hot gases and pressure contained in the barrel, propelling the bullet, without damage.  The Krag action bolt of course has one locking lug.  This has limited .30-40 Government loadings to maximum pressures of 40-45,000 lbs per square inch.  There are numerous loads, meeting all needs, that are safe in the Krag.  Norwegian Krags, in 6.5x55mm, are considered to have better steel and metallurgy than U.S. Krags.  However, they are also accepted as being a weaker action than the Swedish Mausers, with dual locking lugs, when it comes to reloading.
  The 1903 Springfield uses the rimless .30-06 cartridge which headspaces on the case shoulder.  The 1903 barrel-chamber does not fully support the cartridge case in the head area.  A case failure allows hot gasses and high pressure to enter the action.  The early (LOW #) 1903 actions are hard and brittle and more subject to burst or shatter in the event of a cartridge case failure.  During WWI, a double heat-treat was instituted that left the steel more elastic so that receivers were less likely to fail.