Ammunition

Barrel Twist in the AR-15

Barrel Twist AR15

Since the early days of firearm building, armorers noted that if they imparted spin to the projectile that it greatly enhanced in-flight stability and accuracy. The earliest rifles had numerous bands of metal that were forged together and twisted to create the helical shape of the rifle groves. As machining processes were developed and refined, hammer forged barrels became popular as they were much stronger and much more precise. Rifle twist is represented with a “1;” a colon; and another number, such as 1:7, 1:9, 1:10, 1:12, etc. The second number is the length in inches that it takes for the grooves to make one complete revolution. Thus, a 1:10 twist rifle barrel makes a complete 360 degree revolution in 10 inches. A 1:7 rifle barrel on the other hand makes a complete turn in only 7 inches, giving it a much tighter faster rate of twist (and consequently a greater RPM to the bullet).

The Greenhill Formula, developed by Sir Alfred George Greenhill, lays out the mathematics for computing the optimum spin and rifle twist necessary to stabilize a bullet. His most basic calculation is where C = 150 (or 180 for muzzle velocities greater than 2,800 fps) D = bullet caliber (in inches) L = bullet length (in inches) and SG = bullet’s specific gravity (10.9 for most lead bullets). For lead core bullets, the second half of the equation is disregarded as the value of the square root of 10.9/10.9 is 1, however the value will need to be calculated for steel core, steel jacketed, or frangible bullets as their specific gravity will vary. Because of the high muzzle velocity of most 5.56/.223 rounds, C should be set to equal 180 in the above formula.

What does all of this mean? For most shooters, not much. For our purposes, it means we can determine the appropriate twist based off of the bullet weight for a given caliber since bullet length is generally a function of the combination of bullet weight and caliber. Having said that, we’re not going to delve any deeper into the mathematics of calculating the optimal barrel twist for various bullet designs. Instead, we’ll lay out the basics and give you some good guidelines to go by when figuring whether or not your AR-15 barrel will stabilize a given round.

In general, you want a faster twist (lower second number) for heavier bullets. Firing lighter bullets through a fast twist barrel can over spin them, causing inaccuracy from overstability and/or spin induced drift. Overstability occurs primarily in light weight projectiles fired from a fast twist AR barrel and causes the bullet nose to remain at a high angle of attack during the descent phase of the flight trajectory, due to extreme gyroscopic stability. Extremely light weight, thin jacketed varmint rounds, that are overspun past 300,000 RPM, can even fly apart from the immense centrifugal forces imparted by the bullet spin.

For 5.56/.223 bullets weighing between 35 and 50 grains, you can use a 1:12 or 1:14 twist. 1:9 (probably the most common twist found in AR rifles) and 1:10 are good, moderate twist rates that are capable of stabilizing bullets weighing from 45 to 69 and even 70 grain bullets. For the heaviest 5.56/.223 bullets, you will need a 1:7 to 1:8 twist barrel in order to reliably stabilize bullets weighting between 69 and 90 grains.

There are some odd barrels out there being used to fire heavily customized .223 loads. Some custom barrels are available in a 1:6.5 twist and are capable of stabilizing 100 grain bullets, though that weight is not very common and difficult, if not impossible, to find. Extremely high velocity loads firing a bullet weighing 55 grains or less, at speeds exceeding 4,000 feet per second, require a very slow twist rate of 1:15 to 1:16.

Most shooters find that a 1:9 twist barrel meets their needs quite well, but if you’re going to be firing heavier match loads, or lighter, faster varmint rounds, you’ll need to search for a barrel with a more appropriate twist rate.

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Comments (35)

  1. Physics formulas do not support the over stabilization myth. It is a myth that bullets are over stabilized from to much rotation. I have heard a million + stories about bullets flying apart during flight. In 40 years of reloading and shooting I have never had any bullet come apart in flight due to over rotation. I would like to know of a specific bullet and twist rate I can test for myself to verify this popular myth.
    What I have seen time after time is bullets that wobble or tumble because they do not have enough rpm to stabilize them.

    Submitting this comment is a total pain in the a**

  2. Follow up on unanswered question about ‘over-stabilization’. To my understanding that term applies to high velocity cartridges which are then loaded with flyweight bullets. Examples to consider are using a 1:10 barrel for a 22-250 pushing a 40gr instead of a 1:12 barrel; (maybe) a .243 Win pushing a 60gr thru a 1:10 barrel; a .264 WinMag pushing a 77gr thru a 1:10 twist barrel. These are all examples of light-for-caliber bullets going through barrels rifled for distinctly heavier weights. 55s from a 1:7 shouldn’t be an over-stabilization issue, these are not flyweights for caliber.

    To conclude, over-stabilization refers to the phenomenon of jacket separation due to experiencing higher rotational velocity than the bullet Mfr designed it for. My baseline reference: spin a bowling ball fast enough it too will separate before striking the target, used within its design limits it is quite effective in penetrating that target.

  3. First things first, a 4″ group at 50yds indicates a problem that may need close inspection by an AR15 specialist; 10″ groups with a different ammo at 50yd seems to confirm the suspicion of a problem AND signals the usual dilemma of rifle shooting–no two loads shoot exactly the same from a given weapon.

    My guess would be that you have a minute burr somewhere along the bore or at the muzzle. I say this based on reading about someone else’s problem with an M1 Garand that wouldn’t print well, sold it cheap to a sharp eyed buyer seeing the crown needed work; recrowned rifle then shot tightly so the writer claimed. Too, frangible bullets are often made with thinner jackets–more prone to be torn by bore/crown imperfections.

    Applying these ideas: Swap your upper with that of a buddy’s rifle and see how you group with YOUR ammo. From that test result you can determine what portion of the problem is shooter/equipment and what is ammo. Then you have good info to pursue 1) professional help with the bore/crown, 2) purchase of a new barrel, 3) attempt a DIY project from which you will become infinitely more familiar with your weapon and a more knowledgeable shooter, or to buy different ammo.

  4. Sorry about the delay in response. So more specifically the led group will be about a 4 inch diameter and right in the middle of the target. The frangible group will open up to 10 to 12 inch diameter and mostly centered. This is all from the 50. I sighted the rifle from the 25 with lead. When I shot frang from the 25 the result was similar just the spread of the grouping was a little tighter from the 25 than the 50. Is the 1:7 twist over stabilizing the less dense frang round? If so would a heavier frang round perform better?

  5. Andrew, let’s be concise. Is your AR spraying the 55gr frangibles off the paper like a shotgun pattern or does it simply group them quite some distance from the bullseye (which happens to be off the paper)?

    It’s not unusual for a rifle or carbine to group well with two or more loads, but for them to cluster all load groups around the same Point of Impact is unusual. I used to roll my own in 30-06 and tested them in four different rifles. Consistently <1.5" groups though they did not center my handloads close to what the rifles were sighted for.

    Back to your dilemma, print it at 25yd and 50yd to see what is going on. As stated in a prior post, use what you think is a good, consistent shooting methodology in terms of bore maintenance, trigger control, etc.

    Best of luck. Repost for an update about your findings.

  6. Why does my Colt AR15 with 1:7 twist shoot a 55 grain lead round perfectly but throw a 55 grain frangible round off the target? Thanks for any insight you can provide

  7. A 1:9 twist will shoot most 62 grain (i.e. SS-109 or XM-855) bullets every bit as well as a 1:7 twist. The main reason the military switched to a 1:7 twist for the M16A2 rifle and the new SS-109 projectile was due to the much greater length of the tracer bullet used for that series of ammo. Due to the space required for the tracer compound in the base of the bullet, the 62 grain tracer bullet is much longer than the ball round. You will find that the M856 tracer round shoots poorly in a 1:9 twist barrel. I have yet to try it in a 1:8. I am awaiting an opportunity to shoot it through my new 1:8 RRA barrel somewhere other than my home range, as tracer rounds are not allowed there.

  8. I own a Bushmaster A2 Patrolman with a 1/9 twist and a Colt LE6920 with a 1/7 twist. The only difference I have noted is the 1/7 twist is better suited for heavier projectiles such as a 77 grain bullet. In comparison, the 1/9 has variably no limitations. It can fire the larger projectiles, just not as accurately. In a SHTF scenario, I would use either AR-15 platform and know it was suited for the job. Blessings of liberty to you all!

  9. Sorry for being away for so long. I have a new job that has been killing me with 50 – 60 hour weeks–very few trips to the home office to check messages, much less have energy to respond.

    (21) Don, that’s the way the cookie crumbles. Theoretically, the two barrels should shoot the 62gr equally well; the 1:7 should shoot accurately heavier weights than the 1:9. It is commonly agreed that 69 grains is the separating line between using those two twist rates.

    As a starting point try a load or two with 69gr bullets and compare accuracy. Then move up to 75gr–you should see at that point the 1:7 is still flying straight whereas the 1:9 isn’t adequately stabilizing that long-for-caliber bullet and its groups will be notably larger than mid- and lower-weight bullets print. Expect the difference to get bigger as you climb the ladder of bullet weights.

    (22)Nathan, you should be able to swap out the barrels. Now, unless you have the ‘smithing skills to do so you would need a professional to do it for you. Consult with the shop you plan to use to make sure you buy the correct barrel in every detail.

    Getting a whole new upper is a different question. That choice opens the door to being able to jump back and forth between specific formats/calibers–an important consideration if one upper is a long, heavy varmint barrel and the other is a wispy thin hunting rig.

    (24)John, I feel for you man!! Such a dilemma and your wife can’t understand why you “have to go to the range” again. Wasn’t last weekend a ‘have to’ trip as well?

    Answer these, how high is your scope above the 300 BO barrel? How high is the scope above the barrel on the other rigs you refer to? We’re talking centerline to centerline as best you can determine it. Then go to JMB Ballistics on the web and use that free ballistic calculator to see what you rigs are doing.

    Let me see if I can pare it down to short and easy by comparing the same rifle/cartridge in two scope mounting situations: A low mounted scope zeroed at 25yd and on paper at 100yd has a long shallow hill to climb making it up to “zero” and archs for a relatively short distance above the line of sight before coming back down across line of sight–the second “zero” (at whatever that distance turns out to be). A high mounted scope zeroed at 25yd has a steeper hill to climb in making that first crossing of the line of sight, and thus climbs higher above the line of sight in its arch before finally falling through the line of sight the second time (at a distance greater than the low mounted scope).

    I developed an illustration of this point tracing the curved blade of a chef’s knife and a straight line to help show a friend what I was talking about. Just as the knife’s curved cutting edge does not change so does the trajectory of a bullet remain unchanged regardless of scope height. Managing and manipulating scope height to your advantage is the key.

    *******

    My heretical opinion is that the higher a scope sits the better as long as adequate cheek contact on the comb is still present. Up close it might mean only 1/4″ difference in flight path, midrange it might mean 1/2″ to 3/4″ difference and at the far end the higher mount often adds 50+ yards to the effective dead-on hold you can muster with a rig. If you are one to question this point try this experiment: fill 2 milk jugs with water. Place them both at 50 yards. Shoot one in the little circle indent on the side, watch the jug explode. Shoot the other one about 3/4″ outside of that indented circle, watch that one explode too! Lesson applied: if the kill zone on a deer is milk jug sized then eplain to me what difference 3/4″ makes. Hmmmm?

  10. hi i have the 300 blackout and was sighting in my new scope which i started at 25 yds after getting a tight group i tried shooting it at 100 yrds my bullet was about 13 inches to high. deos anybody know why this cal. does this all my other rifles sighted at 25 yds would still be on paper at 100 yds.
    this is what i have CMMG SST 300 BLK
    16″barrel 1:8 twist
    Ammo: remington premier match 125 grain 2215 fps

  11. Pingback: 6.8 spc
  12. Big John, I own two AR’s, one with a 1:9 twist and one with a 1:7 twist. The 1:7 is an Adcor Piston driven AR. I shoot both 62g SS109 and 55g M193 ball. I find that the Di 1:9 twist actually shoots the 62g better than the 1:7 piston. Maybe it is just the way I shoot but I wanted to know if you thought the 62g is a better for for one or the other?

  13. In a word, no. Shoot the weight that on-paper groups tell you are the most accurate.

    The only way to know that is to punch holes at the range. Use what you think is good protocol and technique, be consistent. I have now and have had in the past rifles with which the second group is bigger than the first unless the bore is brushed and dry swabbed.

    The point to that report being: Don’t compare the first group of the day from a clean bore with the last group 50 shots later with a filthy one. It’s up to you what ‘good protocol’ and ‘consistent’ are. To me those concepts include sandbags fore and aft on a concrete table; sitting under shade with a well lit target, but not harshly so; a shot every 2 – 4 minutes (shooting is my ‘golf’, I take my time); eyes on the target, but focus on what the trigger feels like–caress and coax it like when unhooking a wild trout, bear-pawing it is poor technique.

    Only after doing all of that will you be able to say, “X shoots straighter than Y, and the paper targets are in my notebook.” Go to it Buddy and get back to us with which weight shoots better for you!

    As a side note, I had a ’17 Enfield in full military dress 20-odd yeears ago that simply wouldn’t shoot anything straight until I tried Sierra 190 Mkg and the 200 over H380–amazing!! Any other brand of 190/200 match grade bulletss simply sprayed 2+” groups. So, you cannot guess in advance what will or won’t shoot straight until you’ve sacrificed paper on the altars at the range.

  14. Thank you very much for your time and explanation. That is exactly what I wanted to hear. Sounds like you know what you are talking about Big John…

  15. You are in very good company to be running those down the tube. Let me explain.

    Damage? Damage?!?!

    Every single shot out the barrel of every single firearm “damages” it to the extent that one shot less is left in its practical service life. Maybe your question should be, “Does this bullet/twist combo accelerate barrel wear?”

    Read the original article, the bullet you want to use is well within the design intentions of the 1:9 twist rate. You should expect normal accuracy and barrel life (in terms of # of rounds fired).

    If you handload and are trying to reach north of 3500fps you can expect ‘damage’ in the sense of a very short barrel life. If using factory loads of any bullet weight you should be in good shape for a long, long time. Most sources recommend staying away from anything over 69gr when using 1:9; my AR has fading accuracy with that weight so I’ve never chambered anything heavier.

    Some people might go so far as to say 55 – 63gr bullets are the bread and butter of the 5.56, and that may be quite near bullseye, just look at retailers’ offerings, about half to 2/3 of their stock is within that range.

  16. I have a Windham Weaponry AR-15 1/9 Twist with a 16 inch Barrel that says .223/556 Nato. Will a 62 Grain Bullet SS109 damage the Barrel of my gun? I normally shot 55 Grain Federal. Thanks so much.

  17. As an extension of others’ comments above, going a high enough velocity through a barrel of a given twist rate should stabilize the bullet you are considering. This applies to most any bullet diameter. (“Overstabilization” as some people call it is a different issue typically associated with light-for-caliber bullets which is not your question here.)

    Choose your words carefully, you don’t “have to slow my twist” if you are going to push velocities way up there. For example, running 80gr bullets through a 1:7 barrel should be a straight flying pill within a certain velocity range. But to change barrels to a 1:8 or 1:9 may require inordinate velocities above what the 5.56 case capacity can produce (within SAAMI spec). What you have to consider is the shortened barrel life due to blowing large amounts of burning powder down a small bore. (For a textbook example, see what causes were behind the demise of the .264 Win Mag.)

  18. This has to be one of the best informed sites I’ve read in the past many months!!

    I measure the value of information two ways: grammar and reference sources/data. Everything I’ve read here agrees with what familiar reloading books say and what my reloading experience with 8 different cartridges points to.

    The recent purchase of a S&W M&P 15 has me in the AR game for the first time. It opens the door to activities a bolt 30-06 is not well suited to; its 1:9 twist is supposed to stabilize every bullet appropriate to intended use (coyotes and large cats up through feral pigs and deer). Matching bullet construction the the hunt will be the key.

    Just my opinion, at the point one is pursuing long distance targets with .223 bullets over 80gr the shooter might want to consider the next heavier caliber/cartridge in order to access greater case capacity and the attendant velocity. The caveat being if the LD game being played is specifically to use the AR format at 500+ yd. In that case, bully for all of you!! Put ’em all in the same hole!

    See you on Main Street at the nearest prairie dog town.

  19. Will a 1:9 16 inch barrel stabilize subsonic 5.56 or 223 ammo? Most of the subsonic ammo is 100 gr. Is there a easy way to test a bullet for stabilization before firing it thru a can? Thanks for any information.

  20. I went with the 1:7 twist because I read that’s what the military uses. So you can shoot a wider variety (Heavier) of Ammo? Whether or not this is true.. idk?

  21. Not quite Frank: A higher twist rate actually does not affect the speed of the bullet. The bullet velocity as it exits the muzzle is the result of the amount of powder in the load, barrel length, and bullet weight. It doesn’t matter whether the barrel is a smooth bore, micro-grooved, 4 groove, or whatever (ok, there will be a SLIGHT difference, but the resulting difference in speed is negligible for our purposes here).

    Where barrel twist comes into play is spinning the bullet fast enough so that it stabilizes like a well thrown football, but not so fast that it overstabilizes or overspins. Because of the increased bullet length and other forces which we won’t get into here (CG, moment, etc.), it takes a faster twist rate (lower number, like 1:7 instead of 1:10) and higher RPM to stabilize a longer (heavier) bullet in flight.

    All things being equal, the heavier the bullet the slower the muzzle velocity since it takes more energy to push the projectile. Velocity is important, because the faster the bullet is traveling down the barrel the faster it will spin as it engages the rifling. If a heavier bullet is being propelled at a lower velocity, we need a faster twist rate to get it spinning at the right RPM by the time it exits the barrel. In the case that you have two bullets weighing the same but being fired at different velocities due to more or less powder in the load, you will need a slower twist rate for the bullet with the higher muzzle velocity.

  22. THANK YOU! I’ve been asking people for a while about this and gotten muffled answers. Here’s my question/Assumption: A higher twist rate will make the round/projectile more accurate but slower in speed? Is that right? A smoothbore will require less powder to push the projectile out to a determined distance? Feel free to ridicule or even ignore this question.

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