.223 twist- one of my favorite subjects...
Here's probably more than you wanted to know about twist rates and the .223!
The original rate for the AR15 with 55 gr FMJ was 1/14, which was changed to 1/12 due to poor performance in sub-zero temps. The 1/12 barrels barely stabilize the bullets, which makes for increased wounding charicteristics. The bullet goes in about an inch, then turns sideways. This is very good if you want to kill something, but not so hot if there is brush or leaves in the way, as they also deflect these slow turning bullets.
There were many stories coming out of Vietnam about putting a whole magazine into a guy hiding behind some brush and having him run off, which led grunts to believe the round sucked. On the other hand, one clear shot into the torso resulted in vitually instant one shot stops, often with horrific exit wounds, which led other grunts to believe that the round was simply awesome.
The two stories exemplify the problem with the 1/12 twist, as they are both true. This was one issue that was dealt with in the design of the M126A2.
The A2 was designed with a 1/9 twist which was then changed to a 1/7 twist to better stabilize some specialty bullets, including one particularly long tracer bullet. Gelatin tests show the new 62gr FMJ to pentrate deeply without the yawing sideways. Wounding charicteristics are different from the rapid twist barrel, more in line with the military line of thinking which involves overloading the enemy's support stucture with lots of casualties.
So what if you use the 55 gr in the 1/9 or 1/7 barrel? Well, if you don't shoot over 400 yards, you'll never notice any difference. Over 400 the quicker twists prove to be a shade less accurate with the lighter bullets because of overstabilization. So what the heck is overstabilization?
Imagine a perfect long bomb football pass. The ball is spinning around it's axis, and as it travels along the arc of it's trajectory, the nose drops and it stays pointed right at the reciever. This is a perfectly stabilized projectile. Now imagine a high school football pass, where the ball's nose stays pointing up at the sky and the ball starts to wobble the further along it goes. This ball is spinning so fast that it can't turn downwards, and as it starts to drop it actually ends up with some sideways motion and wobble due to air drag. An overstabilized bullet at long range has the same problem, and this is why it is less accurate. Either pass is adequate at short range, but at a distance the nuances kick in and accuracy suffers.
For your application at 200-300 yards, either twist will do. If you intend on changing bulllets at some later time, just be aware that the heavier 60 grain and up bullets will not stabilize at all from a 1/12 twist barrel, they'll be going sideways at fifty feet. Bushmaster picked the 1/9 twist as the perfect compromise rate because it will stabilize anything up to about 80 grains and not have as much tendancy to overstabilize at shorter range. It makes it easy to pick bullets, ligher ones for shorter ranges, heavier ones for longer ranges. I'd recommend a 1/9 barrel for your rifle.
Hope this helps.
