Tyler Kee asked (paraphrasing here):
So what’s all this “barrel twist” hoopajoop? Does barrel twist impact my accuracy? And if so, what twist is best?
Continuing with my “AR for Dummies” kick, I figured this would be a great opportunity to discuss what is probably the most confusing part of buying an AR-15 and try to make it so simple that even Tyler will understand it. So let’s begin . . .
Modern firearms use a series of grooves inside the barrel called “rifling” to spin the projectiles (bullets) as they pass through the barrel. The bullets are sized to fit (snugly) down the barrel without the raised portions so when the bullet is pushed down a barrel with rifling those spiral designs cut into the side of the bullet and grab hold, forcing it to spin. We express the degree of rifling twist in a barrel by a ratio that describes the number of inches it takes to do one complete revolution (for example, 1:12 means 1 revolution in 12 inches).
There are two main reasons for why rifling is important, and both have to do with accuracy.
First, bullets are imperfect projectiles. No matter how well you make them, one side is always going to have more drag or a little more mass, and that imperfection will drag the flight path of the bullet in one direction or the other. Spinning the bullet forces the imperfections to be applied evenly to each direction, cancelling them out. Mostly.
Second, bullets in the real world don’t operate in a frictionless plane like they do in physics books. There are external factors by the boatload that impact where the round will hit, and spinning the bullet helps reduce the effect of those factors. The reason is that the spinning turns the bullet into a gyroscope, and as we students of physics know a gyroscope likes to stay oriented in a single direction and will resist changes. Changes that result from things like wind, for example.
So, one would assume that the faster you can get the round spinning the happier you’ll be. And, for the most part, you’re absolutely correct. The issue, however, is that if you spin a high velocity round (like the 5.56x45mm NATO) too much it has a tendency to disintegrate when it slams into the wall of air past the muzzle and no longer has the barrel to keep it together.
The other end of the spectrum is also no bueno. Bullets need to be spinning sufficiently fast to get that gyroscopic stabilization going (ever notice how gyroscopes will topple over after they slow down, even if the wheel is still spinning?), and if they aren’t properly stabilized they will begin to tumble. And tumbling is bad. The military ran into this problem when they tried using 1:14 barrels, and they’ve since spun things up quite a bit.
So 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:
- 40gr likes 1:12
- 55gr likes 1:9
- 77gr likes 1:7
Some people think that twist rate is a direct factor of bullet weight. Some people would also be wrong. Thanks to physics, we know that the ideal twist rate for a projectile is determined by the combination of the length of the projectile and the velocity, and not the weight of the projectile. But since we’re dealing with identical materials (lead, copper…) in a fixed diameter (.224 inches) and bullets are usually sold by the weight, we can use weight to determine which bullet is the right size for the rifling we have. Or, alternatively, we can determine what rifling we need based on bullet size.
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.
This video is me running my 5.56 competition rifle for the first time since the Crimson Trace match (where I was running 40th out of 105 or so until I DQ’ed myself, which is why I was a touch cautious this time), and at about 3:55 you can see me hitting targets from 250 to 500 yards using 55gr ammo out of a 1:7 twist Noveske barrel. The 500 yard stuff didn’t go so well, but I still got a hit. Eventually. Point being that 55gr WILL stabilize (mostly) in a 1:7 barrel.
So what’s the answer? For most people, 1:9 is perfect. It will shoot the widest range of bullets, and will be especially good with the cheapest ammunition.
[Email your firearms-related questions to “Ask Foghorn” via guntruth@me.com. Click here to browse previous posts]
Excellent post.
Great article. Especially for a details junkie like me who’s just getting into shooting. Though you call it “AR’s for dummies”, there’s a lot of miscommunication out there. Many people could benefit from reading this.
If 1:7 will stabilize lighter bullets out to ~400 yds, and is preferred for heavier bullets when shooting beyond ~400 yds, what would be the purpose of the 1:9 barrel?
Is the formula presented based on a 16″ barrel?
What then is better for shorter or longer barrels, say 7″ and 20″?
Or 10″ and 24″?
Thank You!
The mathematically proper twist for a typical 55gr round is actually 1:12, as the math below shows:
(150 * sqr(.224) ) / .724 = 12.5
But since the military went to 1:9 so they can stabilize some of the heavier stuff (like 62gr and SS109) 1:9 became the “standard” twist. 1:9 will work perfectly fine for everything in that middle range of bullet weight, from 55gr through 68 or 72gr. 1:7 will still work, but I’ve had some accuracy issues with lighter rounds.
The formula is barrel agnostic.
The military (at least the US Military) NEVER went with 1:9, it went from the M-16A1 with a 1:12 twist to the M16A2 with a 1:7 twist. The reason for that was to be able to stabilize the longer M856 tracer cartridge. 1:9 is a civilian invention that was the proper twist for the standard M855 cartridge since the majority of civilian shooters would never shoot the longer M856.
Unless they get a deal on some match grade 70-grain 5.56 NATO…. I remember hearing from someone that the shorter barrel carbines actually liked it more, in terms of reliably cycling the action. The M855 Ball will work just fine out to 500 yards from a 20″ 1:7 barrel. There aren’t a whole lot of ranges that long where I am, though.
In this example/formula, 150 should be 180 to get to 12.47.
Enjoyed the explanation, thanks
The purpose of 1:9 barrels is for optimum stabilization and accuracy with 55-grain bullets. 55′s are the most common and usually the most affordable 5.56 ammo available, and this is why 1:9 barrels are the de facto standard for AR-type rifles.
As an illustration, the 1:7 SCAR 16S shoots nice tiny cold-barrel groups with 62 and 64-grain 5.56, but its 55-grain groups are considerably less consistent. A good 1:9 AR will be pleasantly (or amazingly) accurate with 55s, but you’ll see a slight accuracy drop-off with the heavier bullets.
.223/5.56 barrels aren’t commonly rifled any faster than 1:7, because that twist rate works best with any heavy bullet that the .223/5.56 cartridge can throw to reasonable velocities.
Faster-twist barrels are possible, but if you try to spin the projectile up to speed too suddenly, the abrupt jump from 0 RPM (for the unfired bullet in the chamber) to over 6,000 RPM for a bullet exiting the muzzle of a hypothetical 1:6″ barrel at 3,000 FPS, can chew up the bullet jacket and damage the rifling.
Roy Weatherby managed to increase the twist rate of his super-magnum rifles without shredding the bullets or causing dangerous pressure spikes, by using ‘twist-gain’ rifling.’ These are complicated and expensive barrels, which start with a very slow twist rate which gets tighter and faster as the bullet moves down the barrel. Even with CNC machining and cold hammer-forged barrel technology, these barrels are rare and costly so you won’t see them on 5.56mm ARs.
If you insist on shooting a 5.56mm bullet at targets beyond 400-500 yards, you need to shoot the heaviest bullets you can find through the longest 1:7 barrel you can find, and hope for the best. Or maybe just switch to a caliber more suited to long-range work, like a 6.8.
Also note that “gain-twist” barrels have a nasty tendency to rip apart the jacket as the rifling changes the angle of engagement. This is the main reason that gain-twist barrels are often limited to use with monolithic bullets that employ driving bands to engage the rifling.
I meant ‘revolutions per second’ not RPM.
Chris, most people don’t realize this fact but bullets have some of the highest rotational rates of any man made object. Your calculation above are a little off, by a factor of about 60.
A 55gr bullet running at 3200 fps in a 1:7 barrel will be rotating at 329,142.86 rpm. Yes, you read that right. 12/7 = 1.714286 rotations per foot of travel, times the velocity of 3200fps = 5485.7143 revolutions per SECOND, times 60 seconds per minute = 329,142.86 revolutions per minute.
In a 1:8 match barrel running 69gr SMK’s @ 2950fps at the muzzle, the rotational rate would be 265,500rpm.
In a 1:9 barrel, a 62gr FMJBT @ 3020fps will spin at 241,600rpm.
I’ve seen hot, home brewed 220 Swift rounds disintegrate not long after leaving the barrel at > 4400fps, I suspect due to centrifugal forces, although I’m sure that going from zero to close to mach 4 in a few milliseconds may have contributed to the bullet’s demise as well.
Hmmm. Here’s a question. How do ‘polygonal barrels’ factor into this?
Polygonal and other types of rifling don’t affect the twist rate. They’re different approaches to “grabbing hold” of the bullet and causing it to rotate.
“Square bottom” rifling grooves tend to be more difficult to clean, load up with copper or lead fouling, etc. They’re easy to cut because making a button broach for the rifling is easy to make with square corners.
Polygonal and other non-square bottom rifling try to improve upon classic rifling cuts by not having deep grooves that retain fouling, or are easier to clean, or are easier to form with hammer-forged barrels.
Nick, I really appreciate your “AR for Dummies” kick. It’s provided me with a lot of good info, or at least the jumping off point to find more good info, in my quest to figure out what I want to do with my first AR.
OT, but worth checking out: Defensive use of an AR-15 in Maine…
http://chronicle.augusta.com/news/crime-courts/2012-08-09/shop-owner-shoots-three-during-burglary
Another demonstration of why the right of the people to keep and bear arms shall not be infringed. And yes, that includes semi-auto rifles and normal capacity magazines.
I’d like to amplify on your article here Nick, and hope you don’t mind too terribly much. Everything you say for the AR platforms is true, with perhaps the 77gr 1:7 twist rate being a tad tight for just 77gr pills. With a 1:7, you could also launch the 80+ gr pills used in single-loading situations. If one is never going to shoot any bullets that cannot be fit through the magazine, I think 1:7 is not needed (but it absolutely will work) and one could gain some more versatility by going with 1:8.
My amplification isn’t so much addressed at the AR but rifling twists in general.
Here’s a more expansive treatment of bullet stabilization:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf
The first thing people need to remember about Greenhill’s formula is that bullet shapes have changed dramatically since Greenhill’s time. Back in Greenhill’s day (late 19th century), bullets were usually cast lead. They didn’t tend towards the highly elongated copper jacketed (or pure copper) pills we’re increasingly seeing today. Greenhill’s formula is a great starting point for lead and classic spitzer pills of a light jacket over a lead core, but it is increasingly insufficient when we start looking at modern VLD/long-range bullets and all-copper hunting bullets (eg, Barnes TSX’s, etc).
Second thing people need to understand about twist rates is that there isn’t “one” magic rate for a particular bullet. There’s a range of twist rates (and the range might be narrow) that will adequately stabilize a particular bullet length/diameter/weight combination at a particular velocity. On the over-spun end, the bullets fragment going downrange as Nick indicates and on the low end, they start a precessional yaw which results in a “keyhole” effect on a paper target downrange.
Another downside to overly tight twists is higher pressures in the chamber & barrel. The 6.8SPC has discovered this in a roundabout way because they started by using barrel twists intended to launch 150gr .270 Winchester pills which are much longer than the approximately 100gr pills used in the 6.8. Now 6.8SPC barrels are coming out with a 1:11 to 1:12 twist and chamber pressures are lower because of it. Some early 6.8 loadings would blow their primers or walk their primers out of the pockets as a result of running too tight a twist.
Lastly, people need to understand that jacket deformation of the bullet results in an increase in group size downrange. As you “spin up” a bullet faster and faster (in RPM), any small error in runout or concentricity will magnify. This is why benchrest shooters (who value a tight group size) will tend towards using the slowest possible twist that will stabilize a bullet under their shooting conditions. The way to think of this: If you’re running a tighter twist than you need, you’re going to get a bigger group size than you could possibly achieve with a slower twist.
By how much? Well, that requires measurement of the deformation of the bullet and a bunch of math. I’ll bottom-line it for people: It could turn a rifle that’s capable of a 0.3″ group into a rifle that can’t get a group less than 0.5″. To your average AR shooter, a 0.5″ group is grounds for skipping circles around other people on the range. For the benchrest shooters, that’s grounds for chewing the ass of their gunsmith when they get back home.
You can either “buy” bullet deformation (ie, the bullets had slight errors in jacket formation or lead core formation from the manufacture), or your bullet can be deformed when loaded into the case, or when exiting the case and jumping the leade, in the barrel, etc. This is why benchrest guys obsess about how their rifle barrels are chambered, and they spend so much time worrying about whether their case necks are concentric with the bullet and whether their necks fit snugly into the chamber, and whether the chamber is coaxial with the bore just forward of the throat. They don’t want bullets deformed in the jump out of the case and into the lands – or rather, they want to minimize this deformation.
The upshot of twist rates is this: If you’re going to shoot one bullet all the time, you can optimize your twist rate pretty well for your shooting conditions. If I’m shooting long range with an AR, I’m going to try to maximize my ballistic coefficient, and this means going to heavier, longer bullets. These will require a tighter twist rate, but if all I’m doing is shooting long range matches with that barrel, then I’ll have a barrel optimized for 77gr Sierra Match Kings (as an example) and my twist rate will be in the 1:8 area. I will generally attempt to reckon the slowest twist rate that will stabilize the bullet over all my expected conditions – and sometimes, I can’t get that from a barrel maker. There are “standard” twists that barrel makers make and if I want a special twist rate, I usually have to talk to a barrel maker who is willing (and able) to produce a funky twist – and then I have to wait a good long time, because most all the barrel makers are backlogged, thanks to the buying frenzy caused by our Greatest Gun Salesman Ever Born in DC.
When you start trying to stabilize a range of bullet lengths and shapes, you start having to make compromises, and an example of this would be running a tight twist to allow you to stabilize the longer VLD pills loaded in an AR will prevent you from being able to shoot 40 to 50gr varmint pills out of the same barrel, or a 1:9 or even 1:10 barrel which allows you to shoot the light varmint pills on prairie dogs and ground squirrels means you can’t launch the 77gr match pills and have them stabilize properly.
The standard military twist rate is 1:7. This best stabilizes the 62 grn. steel core green tip and the 63.7 grn. M856 tracer round, but it is reported that the SEALS are using the Daniel Defense MK-18/HK 416 in 1:7 with 77 grn bullets…of course they can use what they want. The 1:9 twist is kind of a catch-all, as it will stabilize both the 55 grn. and 62 grn bullets adequately if not ideally, and is indeed the best twist rate for civilian/law enforcement use. Good article Nick.
You kinda touched on it, but why not just pick a 1-7″ for everything?
Here is why…
What many people don’t think about is how fast those projectivles are spinning. 1/12 twist is easy to calculate if you don’t believe me (1 foot = one spin)
Most rifle bullets are spinning between 200,000 and 250,000 rpm!
THAT is why a light bullet in a fast twist barrel flies apart in midair.
Split the difference and go 1:8
My 1:8 (Stag 6H) dislikes the 55s, but loves the 68s. @ 100yds it’s 2moa vs .75moa with 55gr Hrn FMJ and 68gr Hrn BTHP. Oddly enough, I’ve found that 55s and 62s are identical in length, as are 68s and 75s.
A few random thoughts:
1) The “ideal” spin rate for a given bullet needs to take into account not just its length, but also the location of its center-of-gravity and center-of-drag. Since that information is usually not easily found, and since different bullets still have similar general shapes, we can make reasonable approximations using characteristics that are easy to measure.
2) It’s typically better to spin the bullet too fast than too slow. An exception to this is when one really pushes the limits of thin-jacketed varmint bullets; this isn’t usually a problem with the .223 but can get interesting with rounds like the .22-250 and .243 (running one of those little 55gr bullets at 4000 fps through a 1:10 barrel already has it spinning at almost 290,000 RPM; push it down a 1:7.5 barrel and it’ll pick up another 100,000 RPM – but not for very long!).
3) Faster-twist rifling can cause higher chamber pressures for a given load, which means that choosing too fast a twist rate can sometimes cause a loss of velocity.
4) The shape of the rifling (conventional, polygonal, “ratcheted”/5R, etc.) really shouldn’t influence the selection of the numerical twist rate.
And for .308 Win and 7.62×51 NATO?
The 308 I believe is better suited to a 1:10 or 1:12 twist rate.
What weight bullet do you intend to shoot? The 1:12 barrel on my Rem700 SPS does just fine with 168gr or 175gr SMKs, but 1:11.25 is considered optimum for bullets in that range.
Nice article Nick!
I have to admit, I have been researching uppers for a while now. I am more than likely going to go with a 1:9. Like Nick said most ammo out there is 55 gn. It is what I will shoot most often, so a 1:7 seems overkill.
Here is a related question. Before I can choose an appropriate twist rate in a rifle, I need to know which bullet I am going to shoot. So, what .223/5.56 bullet is “best” for home defense in a SHTF scenario?
Unlike the military application where wounding someone is desired (two more soldiers remove themselves from combat helping their wounded comrade), in a SHTF scenario I want to immediately stop the attacker/s with one shot ideally. That is the point of a rifle. (Otherwise I might as well use handguns.) That seems to dictate that standard military ammo is not the best choice for self-defense. Am I wrong? What is the best bullet choice? Then I can match up a barrel with appropriate twist.
I’ve been looking in to this too (I’m a newbie). For home use, I’ve seen a couple people suggest something like Hornady 55 gr. VMAX (or ZMAX) .223 ammo: because of the polymer tip, you get good expansion and a “reduced” risk of over-penetration. I’ve also seen people suggest something like Hornady TAP. I’ve been told to avoid FMJ ammo. I’m going to be using it out of a 1:9 twist Mini-14. “tnoutdoors9″ did a pretty convincing video on the ZMAX/VMAX ammo.
I use 55grn. Federal (XM193) or Winchester (Q3131) 5.56mm for LE and home defense With proper shot placement you are good to go. Don’t use .223 in my opinon. YMMV
For those buy in bulk because they want to shoot cheap, a fifty five grain flat base
like the “varmint nightmare” out of a 14 twist is a great way to go. It is capable of three inch groups, or even less less, at three hundred meters if the shooter can read the cross wind. I have mounted hundred dollar Adams and Bennett barrels on ARs and bolt actions and never had a single complaint from anyone whose barrel I switched. In fact. when the barrels were “shot out,” my friends brought them back to be chopped and chambered for another go around. Again, no complaints. A 9 twist is great with 68s and up, but for the tried and true 55 flat back a 14 is the only way to go for maximum accuracy at short to medium ranges.
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I wish I would have read this article when it was younger because I have several questions.
As Nick stated, “55gr WILL stabilize (mostly) in a 1:7 barrel.” He indicated that his 1:7 stabilized 55gr well enough out to 500 yards to hit the target. That seems more than good enough since 55gr is not going to be the first choice for long range shooting anyway (right?).
If I’m right on that first point, then a 1:7 barrel is just fine for 55gr ammo, which is the most common and cheapest available (right?).
But with a 1:7, the shooter also has the option to shoot heavier bullets for longer range.
Also (and here is my biggest assumption), heavier bullets will have better penetration and therefore would be better choices for close and mid-range defensive purposes (right?). My understanding is that 5.56mm isn’t a spectacular penetrator and it seems like maximizing that penetration would be desirable, especially if the bad guy were partially concealed by a light barrier of some kind.
So my current conclusion (possibly entirely wrong) is that 1:7 would be the best choice for someone who:
• wants to shoot less expensive 55gr ammo reasonably well for target practice.
• has no particular interest in bullets less than 55gr.
• would like to play around with long range shooting using heavier bullets (even though he is by no means a serious marksman)
• wants to load the most effective self-defense cartridges.
Please let me know where I’m right and where I’m wrong. Thanks.
I’m with you on this one. I went with 1:7 barrels because I’m not shooting anything less than 55gr. I cannot even find gr less than 55 anyway. I have just found a case of 62gr green tips by Federal/American Eagle.