People who could change whatever they wanted on any gun they wanted proved beyond doubt that a .308 is more accurate than a 30.06.
It is physically impossible to make a .30-06 into a .308 without changing
anything else. (Granted, a chamber insert would do it. But I'll let you look your whole life for a benchrest shooter that uses an insert. I'm not gonna waist my time.)
Yes you can set the barrel back, but then you're still changing the barrel length, harmonics, etc. You can replace the barrel, even with the same brand, it'll still shoot differently. Heck, two different barrels, by the same manufacturer, chambered for the same cartridge, (most likely) will not shoot the same, with the same load. At least a good 99.9% chance at that.
Here's the challenge for any one that wants to take up the test. Build a rifle with a custom grade barrel (Lilja, Shilen, Krieger, etc.). Use any length action you want (since it's a test you don't have to worry about magazine function). Chamber the gun in .308, break the barrel in, and shoot for the best group possible (limited to 200 rounds of trial and error). Then ream the chamber out to .30-06 and repeat the best group possible tests. In testing, have the gun locked down to eliminate shooter error. After these tests are completed, then and only then, would there be an answer to the question. My bet is that they'll shoot the same (within a small range of error and only because we're limiting the load development to save the barrel life for the other tests).
Who's up for the challenge?
What problems might there be with this experiment?
And like I said in my first post, if it works for the OP. (Remember he was talking the difference between .30-06 and .223, for a predator hunting rifle.) If it works for the intended use, why change?
Wyman