Since this Springfield barrel had no rear-sight holes or Krag front-sight base to align, "easy-peasy" for a machinist with a lathe.

1. Machine-off enough metal from a .30-03 or .30-06 barrel-breech so that a .30-40 chamber-reamer can alter the chamber for the Krag cartridge.

2. Re-chamber with the .30-40 reamer.

3. Screw altered barrel into a Krag receiver until it stops against the flange of the action. Mark the barrel breech where the extractor slot is located.

4. Remove the barrel and grind the extractor-slot. (Then polish, draw file, re-crown, blue, or whatever).

5. Install barrel on receiver and add any sight-holes, dovetails, banded front-sights.

Good and safe enough for a 'Hunting' Krag, no Springfield Inspection or standards involved.

There was a time when .30-03 and .30-06 barrels were surplus and plentiful and a practical replacement for a 'roached-out' Krag barrel.
Once a man was set up to do one, he could likely do them quickly by the score, (with likely improvised short cuts, guides, or fixtures).