From what I've read, 1:7 is a desirable ratio in an AR barrel, and 1:12 apparently has a history, if not a place (still learning), but what about 1:9...?  What were the designers thinking when they made that?  Is it a compromise, intended to split the difference between 55 gr and 77 gr bullets...?

(Can't search on a ratio that includes a : )

Thanks

R

I am of the opinion it's a good compromise.  Most bullets are 55 grain and will stabilize fine in the 1/9 twist, along with some of the heavier pills.  Unless you plan on shooting 70+ grain, the 1/9 will be fine.  Even then, some barrels may be good with the slower twist and heavier bullet.  I doubt that the barrel twist is EXACTLY 1/9, so some may be a little faster while others are slower.  To know for sure, you just need to try some of the heavier bullets and see.

Nothing wrong with 1:9 twist; my 6721 is one of my more accurate rifles...

The Colt 6721 barrel is one of the most accurate "off the shelf" chrome-lined, NATO chambered AR-15 barrels that I've tested; but that has nothing to do with it having a 1:9" twist rate.






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Because of the latest (late 1990's) developments in heavy .224" bullets, namely 75 to 80+ grain match bullets, 1/9 twist rates are not guaranteed to shoot them well.

I have a 16" Colt 1/9 twist barrel that Molon referred to above and mine shoots great as well. Any match grade bullet weighing 52 to 69 grains will shoot under an 1" at 100 yards with some load development.

I always suggest a 1/7 to 1/8 twist rate or anything in between like 1/7.5 or 1/7.7 stainless steel match grade barrels. These twist rates shoot light bullets extremely well and can handle the heavies 100% of the time. M262 or similar ammo extends the useful range of AR-15 rifles.

And the 1:9" twist isn't going to cut-it with the latest generation of magazine length bullets . . .






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This is a really good reply - I agree.  When I first select my twist specification, it was 1993.  At that time, 1/7 was the incorrect choice, as ammunition was predominantly 55 gr; even 62 gr was semi-rare.  Match shooters loaded 69 gr.  And 1/9 is still the choice twist if that is all you ever shoot.  I stuck with that twist through the years, because that was the correct choice in the early 1990's - especially since early 1/7 twist barrels apparently were having mixed results (depending on which report you read).  

But as noted - by the late 1990's, the heavier 75 and 80 gr bullets began to seriously enter the scene on the competition scene - and those are too much for 1/9.   Today, now that Mk262 has been developed to copy the match winning ammunition high-power shooters developed.  And with that military release, the general public has also discovered the value of the heavier bullets.  And so, in the strange path of things in life, that wrong 1/7 twist for the 1980's and 1990's, is now closer to the right twist, for the 2010's, as ammunition has evolved.  I was one of the 1/9 diehards, and it took me a while to realize that my "right" decision, had not kept up with ammunition trends.  Today, I can't say I recommend 1/9, in the same way I used to strongly promote it.  Also, I think barrels have just gotten a lot better, and so too has ammunition.  A modern 1/7 twist BBL shoots just decent quality 55 ammo just as well as 1/9 does now.  

If I could hand pick my twist and change them all overnight - all of my BBL's would be exactly as the above reply states.  Either 1/8, 1/7, or one of those twists in between.

Should you find yourself with a 1/9 - it's as fine a barrel - but shooting anything above 69 is iffy.  Also, beware, a 75 stabilized at 200 in 1/9 is not necessarily stabilized at 600.  I learned that one the hard way.  One thing - there is data showing early 1/7 twist barrels burning up MUCH faster than older GI 1/12 twist barrels.  And data showing 1/9 Barrels making it past 10,000 rounds with no accuracy fall-off.  But I have not seen data comparing a modern 1/7 twist BBL longevity to a 1/9 twist longevity.  I have reason to suspect a 1/7 twist barrel burns up faster, and so to be safe, prefer the slowest twist that I know will work.  I shoot 80 gr at 600 yards with a 1/8 twist, and am a big fan of 1/8.

Unless you have excessive cash to pay for heavier ammunition, a 1/9 is fine.

Dave N

Two reasons I can think of:



One is that the 69gr SMK was one of the first proper boattailed match bullets in .224" caliber, dating back to 1984 during the "sub-caliber" ballistics race in High Power and eventually when the AR took over service rifle.



Two, very thin jacketed varmint bullets were still the norm for such calibers and couldn't handle the centrifugal force that a 1:7 twist put upon them. There's less likely to naught chance of that happening in a 1:9.





Part compromise, part evolution.

I'm replying to this particular post but you guys have all been so helpful!  Thanks!  

I don't want to spend a ton on premium ammo... Those years are behind me.  I bought my Core 15 Scout mostly because I wanted to have an AR before they hit the banned list again.  I have some 62 gr Hornady TAP and I'd be interested in going up to 69 gr if that's doable, but I'm not a match shooter.  Good to know I have a decent barrel.  Thanks again.

You have some very good info here from the guys, better then it usually is with someone running one or the other down.  If I had my choice I would probably run 1/8 in most things but almost any of the twists work very well.  I shoot M193 55grain almost exclusively so even a 1:14 twist is fine with me most of the time, and most of my retro builds will have 1/12s.  The 1-9s will stabilize almost anything you can fit in the mags except the longest bullets.  Some 1-9s will even stabilize the heavier 77 grain rounds fine it just depends on the barrel and the bullet length.  A longer barrel will stabilize a bullet better then a shorter barrel will due to the added velocity.  A 1-8 or 1-7 will pretty much shoot anything you can put in the magazine, and the 1-7 will shoot even the longest bullets out there that won't fit in the magazine.  

Just remember with a 1-9 it is probably best to stick with 69 grn or less if you want to be for certain, but you can test out some of the heavier rounds and see how they do in your particular rifle.

It's not weight, but length of the bullet in contact with the rifling that matters.  The newer "monolithic" solid copper bullets are longer at the same weight compared to jacketed lead core bullets.  I have a 1:9 barrel that shoots 75 grain lead core bullets very well, but horribly keyhole the lighter but longer Barnes 70 grain TSX.  62 grain Barnes TSX shoot right at one MOA from that barrel and they are as long as most lead core 70's.

Double tap.

Gun-rag nonsense.



The bullet is stable while it is in the barrel.  Bullets are given a spin by the barrel’s rifling so that they remain stable after they have left the barrel.

[Captain Obvious] Once the bullet has left the barrel,
no part of the bullet is in contact with the rifling. [/Captain Obvious]

It is the over-all length of the bullet (along with some other variables) that determines the twist rate that is required to adequately stabilize the bullet.

As an example, the Hornady 75 grain A-MAX has a bearing surface of 0.291” (and a length of 1.110”).  The “recommended twist for optimum performance” is 1:7.3”.

The Hornady 50 grain V-MAX has a length of 0.772” and a bearing surface of 0.295”, which is 0.004” longer than the bearing surface of the 75 grain A-MAX  yet the “recommended twist for optimum performance” for the 50 grain V-MAX is only 1:10”.



The above figures are from Ballistic Performance of Rifle Bullets by Bryan Litz.



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Very informative post, that clears up much of the contradictory info I had been given.  Thank you.

Da nada.


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Thank you for the correction.  I misspoke. I understand your reasoning and appreciate learning more from you. There is no doubt that the 1:9 barrel I have will not stabilize the long for weight 70 grain all copper Barnes TSX but stabilizes heavier, shorter, lead core bullets.  Berger's twist calculator utilizes the Miller Twist Rule with its focus on the relationship between velocity, BC, diameter, weight, total length and certain atmospheric conditions.  I have come to rely on Berger's calculations.

http://www.bergerbullets.com/twist-rate-calculator/

No doubt.  The 10-shot group pictured below was fired from a 1:9" twist barrel at a distance of 100 yards using the 70 grain TSX.  Precision was terrible and there is obvious key-holing.






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I've probably missed this along the way, but I'm not sure I've noticed how barrel length works with twist.  Does a longer barrel benefit bullet spin...like say a 24" barrel in 1/9 doing a better job than a 16" barrel in 1/9 with a heavy/long bullet?

So many opinions, so many facts. I am not smart, but I have been shooting, and hand reloading, for more years than I care to admit to .(over 40). I have loaded for plinking, long range hunting, and precision target. But, even after thousands of rounds I do not claim to be an expert. But, in my own experience and from what I have learned from some smart people I have shot with over the years is this:  There is no one perfect barrel for all shooting. One must be realist about what they want the firearm to do.

Example: I have .223 rifles in 1-12, 1-10, 1-9, 1-8 and 1-7.  No, I do not have any of those exotic 1-7.5's or any of that crazy specialized stuff. I do have different twists because I load everything from 40 gr. up in .223. Some of my loads get into the heavies ends of the spectrum for .22 caliber and those 1-12 barrels just don't work. On the other hand trying to shoot a 40 gr. varmint bullet out of a 1-7 barrel would be pure folly. It just doesn't work, they do all sorts of crazy things.

Many manufactures wisely put 1-9 barrels on their AR's because they know that most shooters are going to be using 55-62 gr. bullets in those rifles and the 1-9 is best for that weight. I see all these internet gun experts saying the 1-7 is the only barrel to use because the military uses it. Truth is the military first changed to the 1-7 due to a tracer round they were using that was long and needed the 1-7. How many here use tracer rounds? And, true, the heavier bullets do benefit from the 1-7 but again, how many people actually shoot those? If you were to go to the range and check 100 AR shooters you would find that 90% + of them were shooting 55-62 gr. bullets. As for the few that aren't shooting the standard stuff, they may well benefit from a different twist than the 1-9.

I have a small assortment of AR's with the twists listed above. I have to honest say that for general shooting my most favorite and accurate one (of the carbines) uses a 1-9 barrel. Of course I am running 55 gr. bullets in it. I do have two nice heavy barrel target models but they gets fed something different so they uses a 1-7 and a 1-8. The 1-12 rifle has its place shooting the light stuff, which it does quite nicely.

One size does not fit all.  If you can have only one barrel think hard about what you actually are going to shoot in it. Don't rely on the "experts" who spew out all the "facts" about what someone says is best. Be realistic and honest with yourself, then decide. Research, read all you can. Seek out some of the various sites where the serious target shooters hang out. There is some rich sources of information out there.

Take a few minutes and look at this site...lots of good information.  .223 bullets and twist rates

Not so much.  As an example, the 55 grain bullet in M193 will have a gyroscopic stability factor of approximately 4.25 when fired from a 14.5” Colt M4 barrel.  When the same M193 round is fired from a 20” Colt A2 barrel, the gyroscopic stability factor of the 55 grain bullet only increases to approximately 4.27.

While the increased RPM due to the faster velocity will act to increase the bullet stability, the increased velocity also “increases the force applied to the nose of the bullet at the center of pressure and strengthens the overturning aerodynamic torque which actually makes the bullet less stable.”*  These two opposing dynamics are the reason for the miniscule increase in gyroscopic stability factor.


* From Applied Ballistcs For Long-Range Shooting by Bryan Litz


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Does anyone have any info on the 70 gr TSX out of a 10.5 1/7? I seem to remember someone reporting keyholes and bad accuraccy.

Agreed.

Most of my AR's have Colt 1/12" 14.5" lightweight barrels.

90% of my shooting anymore is M-193 55 gr. It WAS a mix of M-193 and Hornady Urban TAP 40 gr. But being that it's no longer available I've cut way back on shooting it.

A lot of AR owners don't realize that the ORIGINAL twist was 1/14".

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While I really had no idea, I'm surprised the factor is that small.  Thanks for that insight.

7?  

7 is in now for super precision.

It was more a joke on why not 10, since he usually does 10 shot groups.   Joke being, since super light bullets are reported to explode in flight sometimes with high twists; and this was a super light bullet; and less than 10 were recorded....  I'm sure only 7 were fired for some reason, but I had to smile at least.

I agree with your post.  1/9 twist barrel works fine for 55 or 62 grain rounds.

More of an availability thing to me, almost all the current/modern/popular/quality barrels are 1:7,
every 1:9 I seem to see is a "cheap" barrel.  No doubt there are good 1:9 out there but all the one's that catch
my eye are 1:7 for whatever reason.  I do have a pretty nice 1:8 from Sionics, those seem to be pretty uncommon.

I had a 1/7 twist RRA barrel for years that shot 55gr and 62gr fine, no issues whatsoever. Currently I only have 1/7 twist barrels and never shoot anything lighter than 55gr. I really like having the option to shoot heavier bullets, especially since I handload.

You can always pick up some Hornady 75gr Match which they made specifically for 1:9 barrels.

Just beware - not every 1/9 will actually stabilize at distance with that.  Learned that the hard way.

My thoughts exactly, the only issue I've had was with some of 55gr Midway Dogtown bullets. On a range trip a few months ago I made up some warmer loads and one of the 10 shot groups only 4 hit the target.

Was wondering why there so much dust kicking up around 50 yards until it dawned on my when I saw the 4 hits. First time that happened to me. The jackets are extremely thin on them. The barrel used was a Spikes CHF Middy.

It was simply a matter of me having only 10 rounds of this ammunition sitting in my range bag one day when I was testing some 77 grain loads from a 1:7" twist barrel.  It took three rounds to get the 40 grain V-MAX zeroed from the 77 grain zero, thus leaving only 7 rounds for a group.

..

Some AR-15 owners don’t realize that the 1:14” twist was shit-canned before the first Colt AR-15 Sporter was ever released on the US commercial market and before the M16 was ever adopted by the US military. This was because the 1:14” twist only produces a gyroscopic stability factor of 1.13 for the M193 projectile at an ambient temperature of 70 degrees F and at an ambient temperature of 32 degrees F, the M193 projectile is unstable from the 1:14” twist.

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All choices are "compromises".

1:9 has particular characteristics, if it meets your needs, then choose it.

Is it the least compromising choice for a modern AR15? No.

Cite? They don't mention twist rate, but do mention the test barrel was 24" on the product page.



Berger's twist rate calculator is giving me an SG of about 1.3-1.4, which should fly fine.



 

Hornady 75s usually are ok but are marginal in a 1:9 barrel. Sierra 69s are a safer choice particularly if you have some combination of long range, cold temperatures, and shorter barrel.

1:9s are ok, a better option really for everything most folks will ever shoot, all the current tacticoolness barrier blind loads, M855, all the 55gr plinking ammo, and all manner of match/target loads to include the 68 & 69 grain types and the Berger 73gr for handloaders.

Barrel companies could end the dilemma by bumping the twist rate up to 1:8.5 which would better stabilize the 77 and 75gr OTMs. For a brief time there were 1:8.5 target barrels, before the introduction of the 80gr bullets which needed the 1:8 which is common today for target barrels.

Top finisher @ camp perry last 20 yrs must have a clue what works................................

Which was what?  




I've shot UMC 45 grain in my Colt 1/7 and it grouped pretty well, not flying apart.  That's probably a bit different than the 40 grain V max though.  Never tried it.  

I only have 1 1/9 barrel.  But I do have to say, it's pretty accurate.  It's a RRA.  My dad was real proud one time he did a 1 holer from the bench at 25 yards with irons.  Which was good for an old man.  It was like 20 rounds and he made one hole.  He was so excited.  Let me see if I can find that pic.  

Point being that if you're not shooting the real heavy stuff, 1/9 is fine.  I know in theory it's not supposed to be the better choice, but the two barrels I've shot with slower twists did seem to really perform you well with lighter bullets.  

Found the pic, OK, so almost 1 hole.    Not too bad for an old guy at 25 (actually meters) and iron sights on a carbine.....

excellent information, thanks for posting this.

Litz' work has never steered me wrong, literally or figuratively.
Any other [foundation] references that you regularly use?

Hatcher's Notebook by Julian S. Hatcher, Major General, U.S.A., Retired, Understanding Ballistics by Robert A. Rinker and The Black Rifle by R. Blake Stevens and Edward C. Ezell should all be required reading before anyone is allowed to post in the technical forums.


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http://www.ar15.com/forums/t_3_16/689489_Range_Report___Hornady_5_56mm_62_Grain_TAP_Barrier.html


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Make mine 1:7


to say " I don't need 1:7 because I only shoot 55 & 62 gr" is like saying " I bought a yugo, because I never need to drive over 55mph"


why would you buy a barrel that can't take advantage of the heavier bullets, which are superior in long range accuracy, and terminal effect

.

My stash is 95% Q3131 and Q3131A.  And most of my shooting is 100 yards and less.  I've shot at 200 and 300 and at the time didn't have issues making hits on torso sized targets.  Personally, if I need to use my AR in a situation, I'm good with M193.  Or M855.   There are better rounds for that job, but whatever.  I've seen and read enough to feel alright with that.  Maybe I'm just thick headed.  But there's no way I'd consider my AR with a 1/9 a yugo compared to my other AR's with faster twists.  I think that's an overstatement.  

I don't completely disagree though.  If you're building a rifle you might as well go 1/8  or 1/7.  But since I have a 1/9 on my "go to" AR at present.  I don't really have a problem with it.  I still wish there were gel tests on the 52, 62, 68, & 69 bthp bullet loadings.  But I don't think I've ever seen a gel test on any of those.

If all you shoot is bulk M193 or M855

It's an excellent choice

at what distances does the better long range accuracy from heavier bullets start to become apparent?  in my experience, the accuracy of a 69gr smk bullet from a 1:9 barrel and of a 77gr smk bullet from a 1:7 barrel is about equal out to 200 yds.