Bart B.
You are correct, I should have specified that there are exceptions to the mass/length equation. And you are correct about improper fluting not helping accuracy at all, but that doesn't stop people from thinking it does.
In terms of heating and decreasing harmonic frequency, I can't prove it, but I can say that logically it should "even out" in terms of keeping a zero at a given range. Like the old Enfields where the muzzle whip allowed ammo with significant SD to still shoot tight at 600 (the bullets weren't taking the same path through the air, but were impacting in tight groups). As the bore heats up, friction increases enough to slow the bullet a little bit, the resonant frequency decreases a little bit, and the the consequent muzzle whip tosses the bullet out a little higher than the earlier faster rounds.
Can't prove it happens that way, but it is a logical explanation of how a muzzle can droop a thousandth of an inch with a 100 degree F change in temp and still shoot tight.
I've been trying for years to come up with an economical way to put an aluminum sleeve over a premium pencil thin barrel. So far I've not been able to do it without either a massive amount of equipment investment or time investment. Let the aluminum act as a big heat sink and the large diameter maintain rigidity, just one more attempt at the great compromise of "light weight maintainable accuracy" dilemma.
Jimro