Id imagine the effectiveness of a longer barrel is related to what cartridge your firing. The longer the barrel, the more time the force from the propellant will have to accelerate the bullet. But, you'll eventually run into diminishing returns, where the propellant pressure will drop and be less than barrel friction among other factors, lowering velocity. So, thinking aloud, a longer barrel should be advantageous in cartridges with more or faster burning propellant, to maximize the time that the force is pushing the bullet.
With some of the thicknesses and materials I've seen on some of the longer barrels (28-30"), I cant imagine the reduced stiffness coming into play enough to affect bullet trajectory. "ICE-PICKS" barrel above, he wanst joking, looked like a truck axle. Mind you we're getting into the ultra-precision category though.
I also wouldn't think you'd need a long barrel for 3-400 yards though, and that a shorter barrel would be nicer to carry around, for hunting or tactical employment. But, 800 and up, where you just plant your rifle down and arent dragging it along, you should be able to reload a cartridge that'll take advantage of a long barrel.
Blah blah blah I know...Basically, what Im thinking is, who cares what barrel length your stock rifle comes with and just get the rifle you want, because you wont notice the +/- in accuracy unless your competing at long range, then barrel length is just one of the many varibles you now have to worry about
