"Mike Irwin has heard this story, too."
I've not just heard the story, Tex.
I've seen the bullets, which are in American Rifleman's internal reference collection. I can't remember for sure, but the rifle may be, too.
The guy apparently brought it into the old building down in DC back in the 1950s or 1960s to see the technical staff and find out why the recoil was so horrendous.
That's when they found out that it had been rechambered, but not rebarreled, for .30-06.
The guy had been shooting surplus military 150-gr. ball through it. The bullets look like what you'd expect to find in a 6.5 caliber round from the turn of the last century, sort of round nosed and LONG.