I'm not so sure about that, I'm very happy with the performance of my 20" 6 Creed shooting my AR next to my buddies 27" 308 shows plenty of advantage , wouldn't want it any shorter though.
.
"Advantage" for what, though?
A 27" .308 is squeezing every last drop out of the round's potential, easily hitting 24" .30-06 velocities with heavier, high B.C. bullets (assuming he's able to use those)
Cutting a .243 or 6 CM to 20" is giving up better than 100 FPS from a 24" gun, and the bullets do not have the mass no matter which way you slice it.
For punching paper, meh, either one will do fine. The smaller cross section of the 6mm bullet probably makes wind drift about a wash with the heavier but larger profile .30 cal pill. Beyond that, drop can be accounted for.
Game gettin'? I'd take a 16" .308 over a 20" .243 every time.
I'm a big fan of shorter barrels whenever possible, but there's a reason my .25-06 is still 24" and I just deal with the extra 7.5" the can adds. .243/6 CM aren't much less overbore, have a powder charge to bore area ratio very close to .300 win mag. They're a little more efficient, but not much. The ratio for .308 is much lower, as is 5.56 (roughly the same as .308, around 600 grs. per sq. in, while .243/6 CM and .300 WM are around 1,000, and the .25-06 ~1,100). As a general rule, the lower that ratio, the less barrel is needed to achieve most of the velocity potential. .45 ACP ratio is about 40-50 grs per sq. in. On the other end of the spectrum, we have the ridiculously overbore rounds like 7mm RUM and .30-378 Weatherby, pushing 1,500 grs. per sq. in.