I moved from from MA to TX a couple years ago, and I'm looking to get my first post-postban barrel.

I shot almost entirely 55gr ammo. Mostly XM193 with a little AE223.

Am I going to notice any accuracy differences by going from 1/9 to a 1/7 twist?

On the basis of the twist rate alone? No. There is nothing inherently accurate or inaccurate about a 1 in 9 or a 1 in 7 twist.

I just bought a brand new Compass Lake Engineering barrel to shoot XM193 and its a 1 in 12 twist. That is all you need for XM193 and 55 grain bullets. In fact unless you go to the Arctic, Anarctic or Siberia 1 in 14 twist is sufficient for 55 grain 5.56/223 bullets.

Why the cold weather concern?  Does the cold affect the ballistics, or is this a reference to the winds?

There are a lot of threads on this and other sites with some pretty good info and test results from various twist rates and ammo types.

The twist should only come into play in terms of precision when you go heavier in bullet weight -- like 69g and up. I believe your precision results will generally have more to do with other factors than twist rate.

I think that every barrel is different.





I have a 1:9 barrel that shoots surplus M193 to 2 MOA, and match ammo to 1 MOA, and can handle 69 gr. BTHP pretty well.





I have a 1:7 barrel that shoots 75 gr. match ammo <1 MOA, but sprays M193 all over the place.

 

Agreed.  Many of my barrels do not agree with the hard and fast rules of arfcom.  Perhaps they will learn.

1:7" twist versus 1:9" twist with 55 grain FMJ Ammunition

Using Prvi M193 ammunition, I did an accuracy comparison firing the 55 grain FMJ load from both a 1:9” twist Colt HBAR and a 1:7” twist Colt HBAR.  Four 10-shot groups were fired from each barrel from a bench-rest at a distance of 100 yards.  The groups from each barrel were over-layed to form 40-shot composite groups.  The mean radii of the composite groups were nearly identical.  







....

Extreme cold will cause the muzzle velocity of the bullet to be lower and may require a slightly faster twist to stabilize it. XM193 seems to be rather temperature-sensitive.

Cold impacts the ballistics. I believe below 0 temperatures were enough to destabilize the M193 in a 1:14 twist.

From my experince, the best do all AR Twist rate is 1/8.My lothar walther barrel build ,will shoot at least MOA, with everything from 40 grain v-max's, all the way up to sierra 69 gr bthp.
my first Ar, a colt 1/7 Hbar, would send 40 grain bullets MObarndoor.

This be the exact information to which I was referring.

99% of the information I now have to work with is thanks to this guy.

By definition, an “unstable” bullet is one that will have a gyroscopic stability factor of less than 1.0 at the muzzle. A typical 40 grain polymer-tipped bullet will have a gyroscopic stability factor of approximately 4.2 when fired from a 20” barrel with a 1:7” twist. [CaptainObvious] 4.2 is not less than 1.0. [/CaptainObvious].


Testing performed by C.E. Harris at Aberdeen Proving Ground and later at Sturm-Ruger has shown that the “overstabilization” claim is largely inconsequential. The testing showed that “overspinning” quality light-weight bullets from a fast twist barrel does not become an issue unless you have a gyroscopic stability factor greater than 5.0 (which would require something along the lines of a 1:6” twist barrel launching a 55 grain bullet at over 3500 fps) or unless firing at an angle greater than 85 degrees.


After the U.S. Military adopted the 1:7” twist for the M16A2, C.E. Harris performed tests comparing the accuracy of light-weight bullets fired from 1:10” twist barrels and 1:7” twist barrels using 52 grain Sierra MatchKings. The accuracy testing was done from 200 yards, (well into the downward slope of the trajectory where the Internet Commando claims that all manner of evil befalls the “overstabilized” bullet), and the accuracy results from the two different twist barrels were also nearly identical.



Quality, modern lightweight bullets (40 to 55 grain) of typical copper-jacket/lead-core construction can shoot superbly from fast twist barrels. Typical 55 FMJ bullets do not fall into the quality category.


The group pictured below was fired from a 1:7" twist Noveske barrel using 40 grain V-MAX bullets.






The 10-shot group pictured below was fired from a distance of 100 yards from a Noveske barreled AR-15 with a 1:7” twist using Sierra 55 grain BlitzKings.






For the Internet Commandos in our viewing audience,  the 3-shot group pictured below was fired from the same 1:7” twist barrel, again using 55 grain BlitKings.






….

So it sounds like, for my use, the 1:7 vs 1:9 isn't going to make a noticeable difference, though there might be an slight advantage to the 1:7 that will be so small I'll never notice.

Thanks for all the info.

It's because of the density of the air. The denser it is the greater the aerodynamic forces that are acting on the bullet trying to make it tumble. The spin of the bullet is what overcomes that overturning force but with a 1/14 twist it's on the edge in temperate climates. When you take it to Arctic places where it's cold, the density of the air exerts a force on the bullet as if flies through the air, that is too much for the spin to stabilize the bullet in nose forward flight. If you spin it faster (with a 1/12 or a 1/7 twist) the bullets gyroscopic stability becomes more rigid and it's enough to overcome the forces that make the bullet want to tumble.

B

Yup.  The 52 grain MatchKings do quite well from 1:7" twist barrels.  While the group pictured below was fired from a distance of only 50 yards, it's a 10-shot group fired from a chrome lined, NATO chambered Colt HBAR with a 1:7" twist using 52 grain Sierra MatchKings.








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Arndog86 - bpm990d nailed it, air density.

Below is information from the former owner of Amralite, Ret. Lt. Col. Mark Westrom on the subject.
The Facts Behind the 1:7 inch twist AR barrel
There's a lot of buzz around that the 7 inch twist barrel is just
the thing for the AR family and that a 1:9 inch twist barrel is second
rate.
This isn't actually true, and ArmaLite's President, Mark Westrom,
was deeply involved with the matter while a Civil Servant at the
Armament Command at Rock Island. It's an interesting story.
In the early days of the M16A2 he received a message that
Procurement was set to buy 155,000 M16A1 barrels just when the Army was
switching to 1:7 inch M16A2 barrels. The problem wasn't merely poor
accuracy or logistics; he was concerned that because the 1:12 inch twist
barrels wouldn't stabilize the M855 well enough to prevent wounds that
were likely to raise Geneva Convention complaints, it would make both
legal and logistic sense to settle for a compromise barrel that would
handle both.
He consulted with Col. (Dr.) Marty Fackler, he of the Army's Wound
Ballistics Laboratory, who agreed with him and offered to run some pig
cadaver tests to verify if a problem existed. Westrom sent the proper
request and Fackler confirmed that use of the M855/865 bullets in the
1:12 inch twist left dramatic but somewhat less than effective wounds.
Westrom consulted with ammunition specialists at Picatinny Arsenal
and proposed that even though 1:7 was required "to stabilize the M856
tracer" the 1:9 inch twist rate should be fine for both M193 and M855
ball rounds and would be good enough for the usual purposes of tracers:
close in fire at night. He was surprised when he was told "No, 1:9 is
actually pretty much optimal for the M856 tracer. 8.5" twist actually."
Upon further questioning he was told that 1:7 was used in the M16A2
because it was used in the M249 Squad Automatic Weapon. FN submitted the
M249 with 1:7 inch twist and it won the SAW competition and was thus
produced with that barrel. They didn't know why 1:7 was used in the
M249. (It over stabilized those bullets, which led to decreased long-
range accuracy and reduced barrel life.)
Westrom asked the authorities to call FN to find out why they used the odd 1:7 inch rate.
The next day Picatinny called back and said "You aren't going to
believe this: FN remembered the accusations the US received over the
cruelty of the early 1:14 inch rifling twist of the early M16." They
used 1:7 inch twist so they'd never have to answer that complaint.
In other words, the 1:7 inch rifling twist was a political decision
of a Belgian civilian, not a technical decision at all. As one foot
follows the other, we've gone forward with the odd barrel.
So here's where we are today. 1:9 is great for M193 bullets and to
around 70 grains. 8 inch twist is fine for bullets up to around 77
grains. Higher twist rates are sometimes used for even heavier bullets.
But these bullets aren't those used outside competitive shooting,
and normally loaded one shot at a time. Some military ammunition uses a
77 brain bullet, but that's rare with commercial arms and is VERY hard
on the bolt of the M16.
ArmaLite does make rifles with 1:7 inch twist for those contracts
that require it, but for almost all military, police, and civilian use
it's suboptimal.
Our customers learned this early on. Years ago ArmaLite sold 1:7
inch twist barrels and the market very much disliked them. It took a
long time to sell them. The idea of a chrome-lined 1:7 inch barrel for
match-type isn't very logical, but we'll build them when the customer
demands.
The bottom line is though, that sometimes what the customer wants is
driven by funny advertisements rather than technical facts. This isn't
the only issue that falls into that area.
Westrom's idea is that DOD should switch to the 1:9 inch barrel for
both the a1 and A2 rifles? He was allowed to convene a meeting on the
topic that stunned him. He was savaged for the idea. And this was by
friends. He raises the issue later when another 120,000 1:12 inch
barrels were being bought, to the same end.
But one of his first actions at ArmaLite was to standardize the flexible 1:9 inch rate.

The OP asked about shooting m193 in 1/9 vs 1/7, not  how do match grade Sierras or other commercially available high quality bullets compare in 1/9 vs 1/7.   I'd be interested in seeing the results of a controlled experiment comparing m193 (not a quality bullet compared to MatchKings) being shot out of 1/7 vs 1/9.  It is the quality of the projectile that will determine the group size, not 1/7 vs 1/9, assuming all other parameters are equal.  And let's stretch the range to something more likely, perhaps 200-250 yards.

This is an interesting story and I don't doubt it, but when I ran the measurements of M856 through the Miller Stability Formula it comes out with a Sg of less than 1 with a muzzle velocity of 3000fps. The Sg was .86 for an 8.5 twist and .77 for a 9 twist.

B

Did you even read the replies in this thread?  Because here's exactly what I posted in the 6th reply in this thread . . .


1:7" twist versus 1:9" twist with 55 grain FMJ Ammunition

Using Prvi M193 ammunition, I did an accuracy comparison firing the 55 grain FMJ load from both a 1:9” twist Colt HBAR and a 1:7” twist Colt HBAR.  Four 10-shot groups were fired from each barrel from a bench-rest at a distance of 100 yards.  The groups from each barrel were over-layed to form 40-shot composite groups.  The mean radii of the composite groups were nearly identical.  







....





Why don't you actually contribute something of value to this website and spend some of your money, your time, your equipment and your energy and do just that.  Be sure to shoot 10-shot groups.  I'll be waiting to see your results posted here.


....

Hey man, don't get your panties in a twist.  You show one experiment with 193 ammo and then go on to prove your point using multiple examples of match grade bullets.  I'm good with that if that's the kind of data the OP wants to accept. Hats off to you. I'm not sure what distance the OP is planning on shooting. Perhaps he's shooting prairie dogs as long range.........maybe he's shooting no more than 100 yards.

While your conclusions are correct, (a gyroscopic stabily factor of less than 1 for the the M856 projectile for the mentioned twist rates) your stated gyroscopic stability factors are incorrect as the "Miller" formula is too simple to account for the much lower specific gravity of the M856 projectile compared to a typical lead core/copper jacketed projectile, or even M855.



...

Get back to me when you can post something that remotely resembles valid first-hand data pertaining to the technical subject matter being discussed.


....

Hey Stoner25,
In case you are not aware of it, Molon posts some of the most thorough, valuable data available on accuracy in the AR field.  I am very pleased to see him posting again on AR15.com.  Read his posts carefully, and you will learn a lot.  JMHO.

The 1-7 looses by a hair because a faster twist generally means faster throat erosion. To put an end to the expert ballistic bickering get a 1-8.

I'd prefer to go with thousands of benchrest shooters that have shown over the decades that the slower the twist that you can use for a given blluet weight, the more accurate the combination will be.  You know, that being the average of thousands of rifles, not one guy with several rifles. It's called the law of averages. Then again, if all someone was going to shoot was 193 that might would indicate they're not concerned about extreme accuracy.

Exactly! With everthing I've learned on this site I still read more and post less.

Please show us this "data" of thousands of benchrest shooter's side to side comparisons of shooting the same bullets out of different twist length barrels.

I agree that 1 in 8 is a great middle ground.  Having said that, people should be aware that using bullets under 50 grains in barrels over 18"-20" can result in RPMs that exceed the bullet's integrity.  Most 40 or 45 gr. bullets are rated to somewhere in the 250,000-290,000 RPM range.



BS flag right there.  Geneva convention deals with treatment of POWs; Hague convention deals with "humane bullets", etc.  The USA is a signatory of neither.

I always heard too fast a bullet in a fast twist would leave copper in the barrel

I read the army wanted 1 in 9 twist - they said the faster twist only benefited accuracy past 600 yards - the marines train shooting long range and developed the m16a2 with a 1 in 7 for that reason - the army trained 25 meters close range combat and thought 1in 9 would work better.

here's the doc in pdf of the army study:

http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA168577

I do not dispute the data posted here.  I am confounded, however, by a recent article in the NRA "American Rifleman" regarding the new 62 grain M855A1 bullet and stabilization of it in 1:7.  According to the article, when the round was being publicly demonstrated for purposes of showing that it met accuracy specs, the Army used 1:8 twist because in 1:7 it could not meet the published accuracy.

The author of the article then did similar testing of 1:7 and 1:9 barrels and the new M855A1 and reported that the groups from the 1:9 barrels were reduced by 50% in size compared to 1:7.

I have not analyzed any of that data, nor have I access to it.  Nor have I done my own testing.  I don't have access to M855A1.  Of course, it is not M193, but one would think that given its higher weight and different construction (no lead), it would stabilize even better in 1:7 than M193,  but, for some reason it did not.

Any thoughts?

You can't really "over stabilize" a bullet with a too fast twist.  As I said above, the problem becomes the bullets ripping themselves apart due to centrifugal force if they are spun too fast- RPM being a combination of twist rate and muzzle velocity.  1:7 or 1:8 can get light weight bullets rotating fast enough to exceed their integrity from longer barrels.  

It could be that the 1:7 twist with the longer M855A1 round is exceeding that thresh hold due to the longer jacket of the longer projectile; if the jacket starts to expand laterally, and doesn't do so perfectly uniformly, or if it allows the core to become loose within, you would certainly see accuracy suffer.

I agree with the degrading accuracy effect of high rpm and centrifugal force when it is sufficient to cause jacket separation.  I have a bolt gun with a 26" barrel chambered in 6mm Remington.  That case has sufficient powder capacity to push 55 grain pills to 4,100 fps.  Lightly constructed 55 grain bullets will literally explode enroute, shedding the jacket and never reaching the target.  I've learned which bullets have jackets stout enough to avoid this and still be able to expand violently on varmints, or to dial the velocity back a bit to control that.  Berger bullets will work.  Current Nosler BT's will work.  Some older lots would not. and some other very lightly constructed  6mm bullets in that weight become grenades before getting to the target.

Whether that is the case with M855A1, I don't know.  The reports do not suggest jacket separation, however, merely that accuracy was 50% better with the slower twist.  The reason was not explored and we are left to speculate.  This issue does deserve a bit more study.  I share the philosophy of benchrest shooters who want the slowest twist rate that will stabilize the bullet at the target distance, and no faster.  Highly concentric match grade bullets with J4 precision jackets will have less "wobble" for lack of a more technical term than M193 or bulk bullets like Hornady 55 grain FMJBT cannalure bullets.  Match grade bullet uniformity and balance can forgive many twist "sins."

That is a very interesting read.  I've no seen it in years.  Thank you, sir.  I've downloaded and saved it for future reference.  It does, indeed support the view that 1:9 is a better choice  than 1:7 for optimizing the already "iffy" accuracy of military ball M855 and M193.  .

Yeah I thought about the fact that it's not typical construction. I'll have to look at my copy of McCoy's book to see what it says, if anything about density.

B

Lilja Talks Twist Rates...