It's all about da Twista, Sistah....
Talk about being anal! Here's some info I've garnered over time, for anyone who might have an interest about twist rates & how they apply. Don't know how well this'll hold format, but copy & pasted from my records:
"...the trick is getting the right level of spin on a given round, and thanks to decades of testing we pretty much know what works and what doesn’t.
As a rule of thumb: [lighter likes slower spin]
⦁ 40gr likes 1:12
⦁ 55gr likes 1:9 \
} Ruger AR-556 is 1:8 - ie would theoretically (formula below) prefer ~66gr.
⦁ 77gr likes 1:7 /
A quick look shows that most US producers of AR-15 rifles are chambering them in 5.56×45mm NATO combined with 1 in 8" or 1 in 9" rifling which will stabilize both most of the .223 Remington and the basic 5.56×45mm NATO cartridges. AR builders who intend to shoot 90 grain bullets will probably gravitate to a 1 in 6.5" twist rate rifling."
[90gr would prefer 1:6.5 twist] [Wiki]
"The “rule of thumb” formula for twist rate, developed by George Greenhill, is:
Twist = (C x [Bullet Diameter in Inches, Squared]) / [Length of Bullet in Inches]
Where “C” is 150 when the velocity is under 2,800 feet per second, and 180 when above. There’s some extra bits that are thrown in when you don’t have a lead core projectile, but that’s the basics. This formula can be used to either figure out the ideal twist for a given projectile, or figure out what the ideal length of projectile for a given twist is.
So what is the “right” twist rate to get? What is best? Well, that depends.
If you’re the average shooter and you own an AR-15 for general shooting and/or home defense purposes, you’re probably going to be running either 55gr or 62gr rounds through it. That’s the standard weight for most bulk ammunition (thanks to the insatiable appetite of the military for it), and that stuff is best stabilized by a 1:9 twist barrel. Most “standard” AR-15 rifles come in 1:9 by default.
If you plan on doing some prairie dog hunting and need light, zippy rounds then a 1:12 is probably best instead. It will still stabilize some of the heavier stuff (like 55gr) to a degree, but not for very far.
If you’re on the other end of the spectrum and long range accuracy is your kind of thing, then 1:7 is the twist rate to go with. Heavier (and therefore longer) bullets are more resistant to external factors and have more mass, which means they maintain velocity longer and are accurate at longer ranges. In other words, ideal for long distance shots. 1:7 will stabilize the biggest bullets you can cram in the case and do it well, but it will also stabilize the lighter stuff (like 55gr) out to (in my experience, at least) 300 – 400 yards."
~SRC: Ask Foghorn: Barrel Twist, and Which is Right for Your AR-15? - The Truth About Guns
.223 REM bullet is 0.224" dia
For length of 55gr, use 0.75 to 0.90 for rule-of-thumb
.224² = .050176
<2800fps [*150] = 7.5264 >2800fps [*180] = 9.03168
<2800fps c 150 * .050176 = 7.5264 / 0.75 = 10.0352 <> 0.90 = 8.36267
>2800fps c 180 * .050176 = 9.03168 / 0.75 = 12.042 <> 0.90 = 10.0352
Bullet Velocity [55gr FMJBT]
Lgth <2800fps >2800fps
/0.75 = 10.04 12.04 < Optimum Preferred twist rate
/0.90 = 8.37 10.04 < Optimum Preferred twist rate
Note: Slower twist rates give higher velocities; ie less in-barrel resistance.
Barrel Twist Rate 1:14″ 1:12″ 1:9″ 1:8″ 1:7″ or 1:6.5″
Max Bullet Weight 55gr FB 65gr FB 73gr BT 80gr BT 90gr BT VLD
FB = Flat Base BT = Boat Tail VLD = Very Low Drag *
Table SRC: accurateshooter.com *To maintain velocity - does not group well (accuracy)
With its 1:8 twist, the Ruger AR-556 would prefer 66gr, which is hard to obtain, and expensive. For general plinking, 55gr is fine, 69gr is practically cost prohibitive. But as a price/accuracy compromise, 62gr is likely your best bet.