Proofing with an overpressure charge is a routine thing. Getting good proof loads is somewhat hard today. I imagine that it was back then also. Proofing usually is done at a safe distance or containing device and the trigger is remotely pulled.
Screwing a 30-06 barrel on for one firing would be easy and have predictable pressure. I wish I had thought of that. The one thing it does not do is give you is the results of a new barrel on the Krag. If you install a new barrel and it is out of spec your test with the 03 Springfield barrel is incomplete.
One proof load is thought to make an action stronger by moving the metal till it is properly loaded on all the mating surfaces. Cracks could develop if only one small point of contact is made on the locking lugs over time.