Barrels don't wear out form the projectiles going down them. Imparting spin on a bullet does nothing for wear.
Not exactly true. While rate of twist may not significantly contribute to throat erosion, where most barrel wear occurs, (although, a faster twist does raise pressures slightly), bores and rifling do wear, it just isn't where the majority of the barrel wear will occur first. The faster the twist, the more wear.
It is also interesting to note that no one has bothered to mention velocity in this discussion. I suppose that maximum velocity within pressure limits is assumed. However, remember that velocity is just as important as twist or bullet length (weight). Although twist is described as the distance, expressed in inches, required for the bullet to make one rotation,
the important factor really is the number rotations over a period of time for a given bullet length. So, if you speed the bullet up, less rate of twist is required. Slow the bullet down, more twist is needed to stabilize it. The guys who shoot muzzleloaders (at least the ones that shoot them well) deal with this stuff more directly in projectile and load selection than most other shooters do.
There is also such a thing a too much rotation or "over stabilized" and this diminishes accuracy." A 65 grain bullet fired in a 1:9 AR at 3000 fps is rotating at 240,000 RPM or 4000 rotations per SECOND. At these kinds of rotational speeds it is easy to see how any minor imperfection in the construction of the bullet will have negative effects on accuracy. This is one of the reasons why you want just enough rate of twist to stabilize the bullet but no more than you need because it magnifies bullet imperfections (among other things). MV x (12/twist rate in inches) x 60 = Bullet RPM
In conventional copper jacketed, lead core construction bullets right at 70 grains (give or take one or two grains) is the maximum most 1:9 twist barrels will stabilize.
1:9 is actually a pretty fast rate of twist and probably the most versatile for an AR. Remember that it's not the weight of the bullet that matters; it’s the length and shape. 1:9 will actually stabilize most heavy (or better stated, longer) bullets pretty well, under most conditions, and is much better with the lighter (shorter) stuff.
Bottom line: 1:9 is probably the best bet for the vast majority of AR shooters. There is such a thing as too much rate of twist. The only reason to opt for a 1:7 or 1:8 is if you are exclusively shooting the longest (and slowest) bullets possible.
There is some pretty good information on the subject here:
http://www.armalite.com/images/Tech...OJECTILE WT vs TWIST 120725 Rev 0 (Final).pdf
Just remember, that Greenhill’s equation actually is a little conservative, leaning a little bit on the "faster" rate of twist side to compensate for variable atmospheric conditions. You can usually get by with a little less rate of twist than his equation states.
Another interesting article is:
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/