ORIGINAL: npaden
ORIGINAL: eldeguello
Assuming the barrels are cut, rifled and chambered with equal precision, the heavy is "more accurate" for twpo basic reasons: Its vibration pattern is smaller, so it throws the bullets less off the axis of the bore. It heats more slowly and more evenly, so the muzzle displacement is smaller as you continue shooting.
But the question is "How much more accurate"? Are we talking 1" MOA better or .1" MOA better?
That question is virtually unanswerable. For most of the many reasons so far stated, the heavier barrel is generally considered to be more accurate. If you go to any long range shoots, that is what you're going to see. But that barrel alone is only part of the equation for having a super-accurate rifle. Action work, truing, lapping lugs, ignition timing, bedding, etc. are all part of the picture, along with finiky handloading.
Just because you buy a gun with a heavy barrel is no guarantee it will automatically out-shoot a sporter weight barrel in the same make/model/cartridge combo. It could have the better chance to so so, but it is no certainty. And to try to define the degree of possible improved accuracy is a pie-in-the-sky guess at best.