AR–15 Barrel Twist Explained

By Jerry Kraus published on in Firearms

AR-15 barrels vary greatly between manufacturers and even models in some cases. One of the biggest variables is the “rate of twist.”

I get a great number of questions about this from people who are AR shopping, such as:

  • “What is rate of twist?”
  • “Why should I care about rate of twist?”
  • “What rate of twist should I get in a AR?”
  • And sometimes “I know the AR model I want, what rate of twist and barrel length should I get it in?”

I’m going to answer these questions and more.

rifling

The spiral pattern of the rifling in a cannon adequately demonstrates the mechanics behind putting a spin on the projectile, to increase the projectiles stabilization in flight.

A Bit of Rifling History

First, let’s recap a little history on rifling so you can look like a genius the next time you are done shooting and you are having a drink with your friends…

Anybody who has thrown a football on Thanksgiving knows that when you put a little spin on the football and successfully throw a spiral, your throw is more accurate. The same is true of musket balls and bullets. By putting a spin on the projectile, it helps the projectile stabilize in flight and makes it noticeably more accurate.

Most likely this concept was originally borrowed from archery. The fletching on arrows (feathers for you non-bowhunters) are actually not straight on the rear of the arrow, they are slightly angled. This makes the arrow spin as it flies through the air, making it dangerously more accurate.

How you do this with musket balls and bullets is by rifling the barrel. Rifling was invented in Germany at the end of the 15th century although it really didn’t catch on until the 1800’s. However, in America, before the American Revolution, there were a relatively large number of German immigrants who were quality barrel makers and gunsmiths and knew the true value of WHY to rifle a barrel and, more importantly, HOW to rifle a barrel. Back then, when they started rifling the barrel of a musket, the musket became known as a “rifle.” This led to the American “long rifle” (more commonly called the “Kentucky Rifle”). The Kentucky Rifle had a devastating effect on the British in the American Revolution because it allowed the Americans to shoot the British at distances far beyond the effective accurate range of the British Brown Bess rifles, which were only accurate to about 80 yards.

You can actually watch how gunsmiths from the 1700s rifled barrels by hand at Fort Boonesborough, KY and it is really fascinating to watch. I have been there a few times and seen this, but you should call ahead to check if they are showing the rifling process the day you go. By the way, Fort Boonesborough is the fort Daniel Boone created in Kentucky and is a great place to take your entire family. I grew up watching the Daniel Boone TV series and loved it.

Types of Rifled Barrels

Enough nostalgia, and on to the technicalities. There are three ways to make a rifled barrel:

  • Cold Hammer Forged, which is preferred by many European manufacturers such as H&K
  • Cut rifling
  • Button rifling

Both Cut rifling and Button rifling are much more common, especially in the USA.

All of these rifling techniques create helical rifling by putting helical grooves in the inside of the barrel that forces the bullet to spin along its long axis and usually with a right hand twist. It is measured by how many inches of bullet travel down the barrel it takes the rifling to twist the bullet one full turn. This measurement is called the “rate of twist” and is expressed as a ratio, such as 1:7, which means the bullet spins one full rotation in seven inches of barrel travel. There are other types of riflingthat are less common but they do not apply to the M-16 / M-4 and AR-15 family of rifles.

rifling cutaway

This cutaway of a cannon barrel shows the land and groove pattern of rifling. While this is not an exact replication of rifling in an AR-15 it demonstrates the concept.

Common AR-15 rates of twist vary between 1:7 and 1:12, but why should you care what your rate of twist is, right? The reason is because different bullet weights perform better with certain rates of twist. So if you are just going to go plinking and do short- to medium-range target shooting, you may be happy with 40 grain bullet weights, which would mean a 1:12 rate of twist would be ideal for you.

If you are shooting long range, or want more penetration, you want a heavier bullet in the AR, such as a 62- or preferably a 77-grain. 62-grain bullets prefer a rate of twist around 1:8 and 77-grain bullets weights favor a 1:7 twist rate.

All of this being said, these days finding ANY AR-15 ammo is almost like winning the lottery, and if you are lucky enough to find any at all it is likely going to be the most common bullet weight which is 55-grain (a 1:9 twist rate). Ideal rates of twist for a particular bullet weight are somewhat subjective; however opinions will not vary greatly among the experts.

Bullet Weight: The Ideal Rate of Twist

So to simplify this for you, here is a chart you may want to print:

Bullet Weight Twist
40-Grain 1:12
55-Grain 1:9
62-Grain 1:8 or 1:7
77-Grain 1:7 or 1:8
80-Grain 1:7

As you can see from the chart, the heavier the bullet you want to shoot, the faster the rate of twist should be to most effectively stabilize the bullet. It’s not that you can’t shoot AR ammo that is on one end of the spectrum through a barrel with a twist rate at the other end of the spectrum, it’s just not ideal. You won’t get maximum effectiveness of your ammunition. One thing that can happen is over-stabilization; this occurs when you shoot a bullet through a barrel with too fast of a rate of twist for that particular bullet weight.

Here’s an example: you shoot a 40-grain AR bullet through a barrel with a 1:7 rate of twist; the bullet will over-stabilize and this will make the bullet not fly completely true at longer ranges. But you wouldn’t want to shoot a light AR bullet at long ranges anyway.

So what’s the best all-around rate of twist?

The M16A2 comes with a 1:7, and the military typically shoots bullet weights from 52 grains up to 77 grains, with 62 grains being the most common in combat. Most experts would agree that the best all-around rate of twist would be something in the middle such as a 1:8 or 1:9. I personally like the 1:7 or 1:8 rate of twist because I like to shoot long range in the desert. A downside to this is that it is generally believed that the faster rate of twist means the faster you wear out your barrel. That being said, you should ask yourself what kind of shooting you envision yourself doing and pick a rate of twist accordingly.

Lastly, most manufacturers do not offer different rate of twist options within a particular AR model. Typically, each model line is going to have a given rate of twist and that’s it.

So you should know what your intention is with your prospective new AR purchase beforehand, and select a model that has a barrel with a rate of twist that is consistent with your primary goals for the rifle.

SLRule

Jerry Kraus is a U.S. Army Airborne Infantry veteran and competitive shooter. He has hunted big game in Alaska and Africa. Jerry is a frequent freelance writer published in Soldier of Fortune magazine. Feel free to connect with him on Facebook or LinkedIn

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Comments (37)

  • Spacegunner

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    DB –

    It is so refreshing to see that shooters like you put thought into the physics of what happens between when the trigger is pulled to the point when/where thep bullet stops!

    The best article on barrel-length versus accuracy, versus MV was in American Rifleman many, many years ago. (You may be able to find something through Google.) They had actually taken a perfectly good bolt-action rifle with (I vaguely recall) 24″ barrel in .308 (since it is such a common cartridge), and cut the barrel in 2″ increments. Each time, they recorded group size & MV. There was no significant difference in group size, and the velocity changed dramatically as the barrel was shortened. I forget what length they stopped, but I am sure it was in single-digits.

    Another American Rifleman article (maybe the same one) cited that the optimum (maximum MV) barrel length for .308-size cartridges was 6-7 feet. This was the length where the expanding gasses from the burning powder peaked, and bullet/barrel friction began to take over. In other words, the bullets’ velocity peaked, and its acceleration went to zero.

    Ah yes, the “iron-sight” sight radius!! You are correct that a longer barrel affords a longer sight radius, allowing for more accurate aiming. My Shorty has a sight radius of 14.75″ with the front sight on the gas block; whereas, my Match Rifle has a 29.5″ sight radius with the front sight about 1″ behind the muzzle. Interesting that the MR radius is exactly twice that of the Shorty.

    I doubt that I will be shooting beyond 300 yards with my Shorty; likewise, close-quarters with my Match Rifle. That is why I have different barrel lengths, and the beauty of the AR platform, and the vast availability of different optics sights.

    If I was to “build” a .308 on an AR platform, I would start with a 20″ barrel (18″ at the very least) with an optic sight as primary & back-up iron sights (BUIS) on the gas block. The gas block is 2″ farther forward on the 18-20″ barrels. One last thought on the .308 AR: muzzle blast! A shorter barrel – even going from a 20″ to 18″ has significant differences in audible & felt muzzle blast. This is moreso in the .308 than .223 because more powder is being burned beyon the muzzle, and the .308 has about 1.75 times more powder than the .223.

    I think it was Elmer Keith who stated emphatically, “Bring enough gun . . .”

    Reply

  • DB Cooper

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    Spacegunner,
    Interesting post. Can you give a link to the barrel length studies? I’d really like to read it. I’d like to see what the theoretical optimal length for a .308.

    I think people are confusing Barrel length with accuracy and the ability to properly aim the weapon at distance with Iron Sights (plastic now). If using iron sights a longer barrel enables you to attain a more accurate site picture but is meaningless with optics.

    Your comment on powder burn time and its affect on velocity is dead on. Especially with smaller caliber weapons like the 223/556 where higher velocity equates to lethality and range. A rifle shooting a pistol round will have twice+ the engagement range of the same round in a pistol.

    Years ago I was rabbit hunting with a friend that was extremely good with pistols. He was carrying a .22 cal pistol (using 22LR) and shot a raccoon at a range of 20′. He shot it 5 times to no affect before I shot it once with a .22 rifle killing it. Upon skinning the animal for the pelt we found all five of his shots penetrated only the skin.

    Reply

  • Spacegunner

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    Many experiments & published reports/articles have proven that barrel length has very little to to with the accuracy of a firearm. It does, however, affect the muzzle velocity proportional to the length of the barrel. Given a specific load (primer, powder, bullet) .223/5.56 with a 16″ barrel will have about 200-300 FPS less MV than a 20″ barrel, a 24″ will have 100-200 FPS faster than the 20″, and so on (depending on bullet weight & burn-rate of the powder). Other experiments have shown that a .223/5.56 would maximize its MV with a barrel around 4-5 feet long.

    In your case, a 16″ barrel would give a MV ~2,700 FPS with a 60-grain bullet. I would expect at least 1 MoA groups if the barrel is made properly. My 16″ “Shorty” shoots 0.5 -1 MoA with whatever I feed through it – even factory/surplus loads. The twist is 1:9 (I have another upper receiver with a 24″ – 1:9″ as well; the both shoot 52-75 grain bullets equally as well.). Same goes with my 26″ – 1:9″ bolt rifle.

    Unless you shoot the >75-grain bullets, 1:9″ will suit you just fine; the 1:7″ & 1:8″ will allow you to shoot the longest/heaviest up to & including the various 80-graine bullets. As posted earlier, my 1:8 Match Rifle barrel shoots the 80-grain Hornady A-Max bullet just fine. If I do my job, the rifle, load will shoot well inside the 12″ 10 ring at 600 yards every time (open sights, prone position with sling).

    I hope this helps. Remember: If you have a good barrel, it will be accurate regardless of length. A faster twist will allow you to shoot heavier/longer bullets without much (if any) detrimental effect on the lighter bullets as well.

    All said, I would rebarrel my “Shorty” with an 18″ – 1:9″ barrel, with no flash hider or muzzle break, to gain the extra MV with the same length as the 16″ barrel with the FH/MB.

    Reply

  • Neil Schmidt

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    Which leads to the question that had not been answered: What is the relationship to bullet weight/twist rate as applied to barrel length. How much is accuracy affected by not optimizing bullet weight/twist rate to the best barrel length? We all know that heavier bullets buck wind better, but heavier bullets also drop faster than lighter bullets due to gravity and due to drops in muzzle velocity. Shorter barrels also affect muzzle velocity. So……..what is the best balance here?

    I would like to choose a barrel length in an AR-15 that would use a 60-grain bullet. According to the article, the twist rate should be 1:7 or 1:8, but what barrel length would optimize this combination? Note: I have never shot an AR before, having been a long-barreled precision rifle shooter for the past 20 years. I don’t want to buy an AR that has a 16″ barrel unless the barrel has the correct twist and will provide some sort of decent accuracy and ballistic coefficient as would, say, a 20″ barrel. If the 16″ barrel would provide MOA, I would be happy. The coyotes would not know the difference.

    Any suggestions?

    Reply

    • Steven

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      You said heavier projectiles fall faster due to gravity? Where did you go to school? Physics prevents what you stated. The other factors have all to do with drop rate, gravity plays the same role irregardless.of weight.

      Reply

    • Scott

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      You are correct gravity has the same effect no matter the weight of an object. The same question could be asked of you though. Where did you get your schooling? Irregardless is the incorrect usage of the word regardless. It is a double negative. Proper English is usually, for most people, an easier subject to learn than physics.

      Reply

  • DB Cooper

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    Paul,
    That’s why I asked about optimal barrel length. An old guy I worked with years ago was a Seabee in WW2 in the pacific. He was a M-2 gunner and he told me they were issued really short barrels (I think he said 28″) and the result was that not all of the powder burned in the barrel and collected in piles in front of the gun which when sufficiently high would flash and blind the gunners. Not something you want to have happen when being attacked. When he complained he was issued a broom to push it out of the way. Yeah right. He told me he tossed the broom and got some boxes of kitchen matches and ever so often he and his assistant gunner would get down and toss a lit match over the sandbags to burn it off. When they were issued the standard barrels the problem stopped.
    Back in the mid 80s a NG unit came to my range with M-2’s and a ton of ammo. It was their old basic load. The rounds had been loaded in 1939! My God were they smoky. After firing a long burst you couldn’t see anything!

    Pleas correct me if I’m wrong but the SS109 bullet needed the extra spin to stabilize it in flight but the faster spin over stabilized it and removed the yawl the bullet needs to be effective.

    Reply

    • Juanito Ibañez

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      DB: Back during Colt’s making design modifications to the M16A1 to create the M16A2 I was in contact with famed Colt engineer Robert “Rob” Roy. We had numerous discussions about the various changes being made, including the barrel and its twist rate.

      What he disclosed to me was that a rate of 1:9″ was sufficient to stabilize the 62gr. projectile of the SS109 NATO (M885 US) round, but to meet the NATO accuracy requirement for the L110 NATO (M856 US) tracer round’s overly-long projectile needed to meet their 800-meter visible burn “length-of-trace” required the 1:7″ twist rate — to-wit: it was length (1-3/32″/27.78mm) and not weight (60.6gr.) that established the M16A2’s 1:7″ twist rate.

      Hence, the rifle was developed and released based upon the need to meet a requirement not needed by 99+% of AR-platform rifle shooters: firing tracer rounds.

      Reply

  • Jim

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    This might explain why my Saiga .308 is so picky for the grain of ammo to give me the reliability of accuracy I need….thanks again.

    PS. I agree with earlier poster above DB Cooper, an article on ar-10’s and ak’s and Saiga twist ratios to higher calibers for enthusiast with preferred twists would be nice… Does this even vary much?

    Are there any other tips on this in popular handgun models?

    Reply

  • Jim

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    Thanks for this FYI. I learned it decades ago in the Military, but had forgotten exactly, especially with the very useful twist ratio to grain charts you supplemented into your well said succinct article…happy faces here…

    Reply

  • Paul Bernett

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    lets back up a little.If im not mistakan the origenal stoner M-16 wase 55gr bullet with a 1 in 14 twist just stbelized. Know for the main ingrediant. Burn time gentelmen ah yes speed was the kee 3200 fpe. in 20 inches. The bullet did not tumble in flite but only after target acquisition. Or lets just say it turned corners. Realy a great and efective killing mechine, But along comes the m4 and now we have a 16 inch barrel, with a 62 gr. bullet with a 1 in 7 twist. Our indy car just put on alot of weight. And it has to have a armor piercing nose for Penatration. What happend? Dont forget about the barrle lingth it”s more important than you might think. WE had to buy back M14″s from other countrys to send to the sand-box we just dident have enough foot pounds at the target BURN TIME Gentalmen–BURN TIME.

    Reply

  • Spacegunner

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    My Spacegun (1:8 twist) aside, my other two .223 upper receiver barrels have 1:9 rifling twist-rates. My bolt-action .223 also has a 1:9 twist. I mostly shoot 55-68 grain bullets through them with no problems whatsoever. All my AR’s, whether in .223 or .22 LR shoot <0.5 MoA with all the bullets & my reloads I put through them. Most factory/surplus ammo shoots <<1 MoA. I reload 99% of what I shoot, however.

    Reply

  • Spacegunner

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    The twist rate of a barrel to the weight of the bullet is only partially true. The true requirement of a particular twist-rate is the length of the bullet. Yes, the heavier the bullet, the longer the bullet, the faster the twist.

    However, the shape of the bullet can make it longer while the weight may the same as the “common” shapes available. Long-range shooters like the long, tapered Very Low-Drag (VLD) bullets. For instance, an 80-grain VLD or A-Max bullet is longer than an 80-grain BTHP Match bullet.

    As has been stated before: Know what application & bullets you are shooting. I shoot 68-grain BTHP Match up to 80-grain A-Max bullets in my Match Rifle (Spacegun) down a 1:8, 26″ barrel, and have no problems with each bullet. I am classified as a Master in NRA High-Power, and shoot 200, 300 & 600 yards with my rifle. I am absolutely sure it outshoots me at 1,000 yards, too.

    As for number of lands/grooves: It makes no difference if the barrel is manufactured by a quality barrel maker. There are several – depending on how much you want to spend, and what you application is (again).

    Reply

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