Re: Barroom argument
Is it physically possible? Yes.
"Just because you can doesn't mean you should."
With a low enough powder charge you could likely do it "safely" (not eating bolt), but you're going to run into a real sharp pressure spike, and likely copper foul like a mother.
I believe P.O. Ackley did quite a bit of testing in this area, and IIRC, over-sized bullets aren't as huge a thing as people would like to think, but they still do increase pressure.
Biggest issue is the throat and neck. As long as there's room for the case neck to expand, and a "funnel" for the bullet to squeeze down it's not so severe. If you don't allow room for the brass to obturate, pressure spikes quickly, however.
In WWII, the Germans had a few squeeze-bore designs, going from 28mm to 20mm (PzB41), and I believe there was a cannon that went from 75mm to 50-55mm, obviously with special ammunition.
As far as 6.5mm in a .25-06, I wouldn't do it on a consistent basis. Maybe one weak-loaded for the "what does this do" factor, but beyond that is a waste of time/effort/money IMO.
Kind of relevant, but one time I tried to cut .30-06 brass down to 7x57mm mauser and load it for a buddy of mine. I used the same process that I do for making 8x57mm for my K98's. However, his new(er) winchester 70 didn't have enough room in the neck to expand the brass (.30-06 brass is thicker at the shoulder, where the 7x57 neck/mouth ended up being, than 7x57 is supposed to be at the mouth). Starting loads blew primers.
Something to think about.
