5subslr5:  Thanks for the kind words - and you bet I was typing at the speed of light (It's how I end up with my typos).

Bushmaster (and the others) settled on 1:9 a while back because at the time there was little interest in the heavy bullets and it did a very good job while lasting a bit longer than 1:7.  Its what the customer's (us) wanted at the time.  Colt AFAIK was the only one producing 1:7 barrels on their commercial guns (just their military ones....).

Until farily recently there was little interest in the heavy rounds for other than long range match shooting.  Now that the terminal capabilities of these rounds are known you'll see more people clammoring for 1:7 (already started on the BM forum).  Until there is enough demand for more than the current 1 run a year they have little incentive to change their expensive tooling.

Yep - ArmaLite is offering some 1:8 barrels and they too will likely change to offering 1:7.  (Recent events in Afghanistan will likely 'encourage' the change.)

I've heard that too.  As I understand it, cut-rifled barrels are exact twist rates because the cutter is precisely controlled.  But button-rifled(?) (and/or broach-rifled?) barrels have some variation in twist rate because the button "slips" some as it's pulled through the barrel.  Wish I could remember where I read that to verify it.  If that's the case and individual rifles vary some above or below the nominal twist rate then that could account for why some "1/9" guns work with 75gr and some don't.  Yaw happens at any velocity with spitzer bullets.  Fragmentation happens with some bullets if they're going fast enough.  Tests have shown that the 68/69/75/77gr bullets fragment at lower velocities and stay above that "magic" frag velocity to longer ranges and are thus "effective" to longer ranges.  The added accuracy provided by them being match bullets is a nice side effect.All TAP [b]except[/b] the 75gr penetrates less than the recommended 12" of gelatin.  If I were you I'd reconsider my choice.Yes, most ARs sold to civilians are 1/9 barrels.  That's because, as Forest noted, until very recently there was little demand for barrels that could shoot the heavy bullets outside of long-range match shooters (who were already buying custom barrels anyway) and the military who needed 1/7 in order to stabilize the very long M856 tracer round.  1/9 worked with everything the typical shooter used, without overspinning light stuff too much.  

That was then.  Now that the 75/77gr loads are popular I believe that in the coming years we'll see more and more ARs sold with 1/7 (or 1/8) twists.  In my mind 1/7 is better because it doesn't limit my ammo choices like 1/9 does.  Well, maybe it excludes the very light, very fast varmint rounds but I have no need for such "specialty" ammo anyway.  I'll gladly give that up to get the superior performance of the heavier bullets.