Excellent article explaining AR barrel twist rates

[DirtyDigz 1,641]

http://www.thetruthaboutguns.com/2012/08/foghorn/ask-foghorn-barrel-twist-and-which-is-right-for-your-ar-15/

 

Take away from the article:

 

As a rule of thumb:

• 40gr likes 1:12
  
• 55gr likes 1:9
  
• 77gr likes 1:7
  

...

 

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.

[Xchange1980 0]

Nice article..always good to learn something new everyday

[BOOMSTICK 6]

Yes... thanks for this. Good info.

[Holeshot 3]

A good base to work off of.

 

However just a quick reminder that you should still shoot YOUR rifle and experiment with different grain bullets to see what it likes best.

 

You may be surprised.

[Krdshrk 3,666]

Pinned

[Shocker 130]

A good base to work off of.

 

However just a quick reminder that you should still shoot YOUR rifle and experiment with different grain bullets to see what it likes best.

 

You may be surprised.

 

Sure, this article is just a starting point. Anything that changes the polar moment of inertia, the center of pressure, the center of gravity all change the dynamics. So that 1:7 that stabilizes a long lead/copper HPBT great might not so so well with a steel core or polymer tip varmint round, even with the same outer dimensions.

[Zell959 40]

I like the article and all of the content sounds correct. That said, his conclusion echoes something I've heard said many times before and still don't get. Inexpensive bulk 55gr rounds are usually going to be around 2MOA on even a really good day. Someone shooting bulk ammo probably isn't in search of really high end accuracy at 300-500 yards anyway, so I don't see how they gain all that much by going with a 1:9, as a 1:7 is generally going to do just fine with 55gr bullets at shorter distances and has the added benefit of making it possible to use heavier bullets for longer distances.

 

If someone is looking for really tight groups with lighter bullets, sure, 1:9 makes sense. But a 1:8 or 1:7 isn't going to struggle to put 55gr wolf FMJ on target at 100 yards-200 yards.

[CAM 5]

good post. the 1/7 sometimes harder to come by. I like the 1/8, easily available in many barrels and gonna serve your ammo needs for range/hunting/defense purposes universally the best of all options.

[Old School 611]

 

 

Sure, this article is just a starting point. Anything that changes the polar moment of inertia, the center of pressure, the center of gravity all change the dynamics. So that 1:7 that stabilizes a long lead/copper HPBT great might not so so well with a steel core or polymer tip varmint round, even with the same outer dimensions.

 

Sounds like the voice of a professional...LOL

 

Please expand on the effect of muzzle velocity vs rate of twist for a given weigght bullet.

 

In my 1:9 with a 69gr bullet I find I have to drive them faster to stabilize them than I do a 55gr.

[DbleTrouble 8]

How about barrel lenght? would length make any difference as far as rotational velocity of the bullet?

[Shocker 130]

Please expand on the effect of muzzle velocity vs rate of twist for a given weigght bullet.

 

Higher MV = faster spin.

 

Longer barrel = higher MV = faster spin

[Old School 611]

Higher MV = faster spin.

 

Longer barrel = higher MV = faster spin

 

So.. that's why if I drive my 69gr bullet faster in a 1:9 twist barrel it stabilizes better than if I drive it slower. Correct?

[Shocker 130]

I would assume so. I don't run PRODAS to try different combinations but generally faster is better (within reason) as it will increase the spin and move the center of pressure aft which reduces the overturning moment.

 

That 69'er is probably marginally stable in your gun at your load so your velocity bump will have results that are easy to see.

[Old School 611]

I would assume so. I don't run PRODAS to try different combinations but generally faster is better (within reason) as it will increase the spin and move the center of pressure aft which reduces the overturning moment.

 

That 69'er is probably marginally stable in your gun at your load so your velocity bump will have results that are easy to see.

 

I'm down to about .75" @ 100 but would like to see a ragged hole like the 55gr pills. I can kick it up at least .5-.75gr and not looking at excessive pressure. Back to the bench...

 

Thanks Dave!

[mipafox 438]

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.

 

Is this guy out of his mind?  The AR15 was designed to fire 55 grain ammo out of a 1:14 twist barrel, it only was changed to 1:12 because it was sometimes understabilized under arctic conditions.  Every AR15 or M16-A1 ever issued by the military was 1:12 and millions of people have used them to qualify with 55 grain ammo at 300 meters or even 500 meters.  Every rifle is different, and every bullet is different, but if there is any expectation of difference in accuracy between 1:12 and 1:9 with 55 grain M193, the small difference would be that 1:12 would be more accurate at any range.

[voyager9 3,284]

[raz-0 1,232]

I would assume so. I don't run PRODAS to try different combinations but generally faster is better (within reason) as it will increase the spin and move the center of pressure aft which reduces the overturning moment.

 

That 69'er is probably marginally stable in your gun at your load so your velocity bump will have results that are easy to see.

 

The article bothers me because while what it says is not untrue, it is basically wrong. Bullet geometry and barrel length factor into it. Pretty much every rifle bullet sold comes with the recommended twist rate on the side of the box. Even for the barrel length and twist specified, it is very often not in line with the greenhill formula. I suspect this is due to the time when he developed the formula vs the advent of high BC bullets. 

 

Actually it's probably safer to say that the article has no really useful information in it outside of getting some context to really research the topic via the search engine of your choice. 

 

 

 

This chart is actually much more useful than that article IMO, especially if you are going the "AR for dummies" route as the article frames itself. Mainly because the odds of being confused are less. A 1:8 barrel with a .223 wylde, 5.56 sam-r, or something like the noveske match mod 0 chamber are very versatile, especially if staying in the 14.5-18 inch range.  

 

For ARs, one thing I would add as another general rule is that you will see slightly better performance form a barrel that gets two full rotations before the bullet exits than one that does less than two. 

 

But to put the article in perspective, there are plenty of people getting some pretty solid accuracy out os a 1:7 chrome lined 5.56 barrel using 50gr varmint rounds. There are also a bunch of people out there getting very good accuracy out of 20" 1:9 barrels with 77gr bullets. 

 

It's not a simple system being modeled, so like shocker said read all that and work up your own loads. If you want to know how to find the best recipe for that, try lots of powders and ladder test your loads. 

[NYMetsFan86 9]

what about 123 grain 7.62x39 ar's? ...maybe 1:7?

[Shawnmoore81 623]

 

 

Sent from my iPad using Tapatalk HD

[dman2112 6]

Is this chart based on a 16" barrel?

[Dark Storm 21]

Great article.  This is why we use 1:9 as the standard twist on our 5.56 guns.  Our 1:9 barrel will out shoot a 1:7 using the same 55 gr ammo.  Yes, our 1:7 will out shoot the 1:9 when using 77gr but we have found most people aren't into shooting $1 a round ammo in their AR's.  Also keep in mind that optimum performance does not come from Federal XM193.  The best performance in the 1:9 barrel was found to be Hornady 55gr V-Max