Cerrosafe is your friend!
You are lucky that you didn't damage yourself or the rifle. Almost everyone would assume that a 1903 Springfield would be chambered in 30-06, but there are rifles out there that have been rechambered without the 'smith having the courtesy to stamp the barrel with the new chamber.
Bearrr264 had a case separation in a Savage 99 that had been rechambered from 300 Sav to 308. When he fired the rifle with a 300 Sav cartridge, the case separated and the escaping gas blew the "cheeks" of the stock out, causing high velocity wood fragments to spray the right side of his face and neck.
