Quoted: Quoted: A 55gr rounds best performance is achieved with the 1/9, the 62gr with 1/8 and so one.   Actually, per "The Black Rifle", chart p. 341 (Courtesy Fabrique Nationale Herstal) 1/12 handles the 55 gr. M193 most accurately, 1/7  handles the 62 gr. SS109, i. e., M855 most accurately, and 1/9  handles both well, but is slightly less accurate than either of the above. No mention is made of 1/8.

This is crap (what's in the book, not you).  It was quoted from someone who either didn't fully understand the concepts, or was "speaking down" to an audience that didn't understand the concepts in depth.

The Ammo FAQ does a much better job explaining the issues and discussing the benefits and liabilities of each twist.  Please take a look.

-Troy

Quoted: Well, I am certainly no expert in bench rest shooting, but in my experience playing around with AR's in a multitude of calibers.....it sure seems that the best groups are usually turned in by barrels leaning on the slow side of the twist window.  Running a twist that is just fast enough to keep the bullet stabalized seems to be the answer for me.  Tightening up the twist seems to give you a broader window of "acceptableness" but is not optimum.

You are very correct, which is why benchrest shooters buy barrels with specific twists for the bullets they intend to use, with the slowest twist that will stabilize that bullet.  As you note, this information was learned and confirmed through years of trial and error, and is widely acknowledged as truth among expert shooters.

However, it should be noted that most people don't need the .2 MOA accuracy that those benchrest folks are looking for.  And that given that only a tiny percentage of ARs have real match-grade barrels, the twist rate is less important with regards to accuracy as long as it stabilizes the bullet.

Sancho,

The information in The Black Rifle and from FN has been simplified into a generalization than unfortunately isn't accurate.  The reason is because both sources are trying to describe a complex relationship in a very few amount of words.  And editors often cut out important words or sentences in search of brevity, without knowing that by doing so, they've altered the meaning of the source.

If you'd like to verify what several of us have told you, you can do the research yourself.  Pickup virtually any book about competition shooting or reloading, and they'll probably cover the topic in great depth.  Why?  Because at that level of shooting, generalizations aren't good enough, and you need to understand the specifics.

Of course, you can choose to believe what you will.  Those folks that have put in the time know the truth.

-Troy

<sigh>

Here are a couple of links for you to read through, so you don't think I'm making this all up.

www.recguns.com/Sources/VIIIB6.html
www.snipercountry.com/hottips/TwistRate.htm
www.loadammo.com/Topics/July01.htm
www.bushmaster.com/faqs/index5.asp


-Troy

Anytime you are trying to choose a twist rate to accomidate two or more bullet lengths, you're going to have to compromise.

Let's use your example.

The "ideal"* twist rate for the SS-109 bullet at typical military velocity is about 1:9.  The ideal twist rate for the long L-110 tracer bullet is about 1:5.5.  Given that these two rounds were expected to be the ammo fired from the Minimi/M249 SAW 99.9% of the time, the twist rate selected for the SAW was [drumroll...]: 1:7, a compromise between the ideal twists for the two bullets.

Who'da thunk it?

So, why are most .223 guns rifled in 1:9 these days?  Because it was realized that few people outside the military fire M856 tracers, and in fact, over 75% of civilian .223 sales was 55gr ammo.  So, the 1:9 twist was a better match for the range of bullet lengths typically fired by civilians, and had other advantages as well, such as longer throat life.

Keep in mind that until very recently, sales of bullets above 69gr were a tiny percentage of the market, being almost excluively used by match shooters, and almost no one used them for combat loads.  Only in the last couple of years has there been a push to find a better load for short-barreled ARs, which lead to the discovery that some of the heavier match loads did extremely well from a terminal ballistic standpoint.  Even now, the program is still in very early stages.

The point of all of this is: determine which loads you anticipate shooting, and then select the twist rate that best matches the needs of those loads.

Hopefully all of this information is enough to help you make that decision.  If not, I don't know what else to give you.

-Troy

* By "ideal", I mean a twist that is fast enough to stabilize the bullet in all climates at least until it goes subsonic, which is important to the military.  For a benchrest person, who isn't likely to be out shooting in arctic conditions, you'll find that "ideal" will usually be a slower twist for the same bullet and velocity.