AR-15 Barrel-Twist Rates

By Glen Zediker published on in Competitive Shooting

A barrel-twist rate is expressed in a chain of numbers that reflects on how far down the barrel a bullet must travel to make one full rotation. So, a 1-8 twist barrel is read as “one turn in eight inches,” and what will happen after 8 inches of its journey is a 360-degree bullet rotation.

The spin imparted to a bullet by the lands or rifling is necessary to stabilize the bullet in flight. What some shooters don’t understand is that it is bullet length, not weight, that determines the amount of rotation needed for stability, though almost always it’s a bullet weight that is associated with a particular twist. The weight/length distinction didn’t matter much until the advent of the “low-drag”–style bullets. These are long, or at least longer, than other bullets of equivalent weights. Longer bullets need more rotation (higher revolutions per minute) to “go to sleep” and fly wobble-free to the target.

52-grain Hornady Match .224-caliber bullet

This is a 52-grain Hornady Match .224-caliber bullet (the actual diameter used in .223 Remington/5.56mm NATO). I have shot a slew of perfect short-course targets with these through a 1-7 twist barrel.

When it comes to choosing a twist rate, I like to err on the quicker side. When it comes to twist rate, “adequate” is a word that makes me a little nervous. I prefer “certain.” I honestly can tell you that I have not seen the ill effects on target from twist rates that were slightly faster than “adequate.” I have, however, seen bullets that needed a little tighter spiral (faster spin) to group properly. Back in the day, which wasn’t really that long ago, there were different bullets appearing to bolster the longer-range potential of .223 Rem. Most decided that 1-8 was adequate to keep an 80-grain bullet flying flat. My experience with that was more nuanced: the Sierra design at that weight, yes; the JLK, maybe. Doing load work-ups on the JLK “VLD” (Very Low Drag) bullets, which are longer than the Sierra, I had a few hit sideways as I was determining the pressure ceiling. I do that by adding a tad more propellant, watching velocities and case condition to guide this little adventure to its end. Groups tightened only when I was close to what I considered a safe maximum charge. To maintain comfort in my world, I want to see stable bullet flights from at least one big step down from “max,” and let’s say that’s a full one grain of propellant.

All in, I think a 1-7 is a better choice than 1-8 for the longer-range shooter, unless that shooter wants to try some of the 90-grain bullets, then the twist rate can be 1-6.5. And, yes, there is a limit. 1-6 blows up bullets. By the way, when a bullet comes apart, it does so from the tail forward.

The point to the foregoing was, and is, not to rely on (high) velocity for bullet stability, just get a little faster barrel twist.

A twist rate that’s too fast for “some” bullet, mostly, may subvert velocity potentials, but it’s slight, in my experience. The reason is pretty simple, as just suggested: it’s more energy needed to push the bullet through extra resistance in the bore.

The original twist rate for the AR-15 was 1-12. That’s pretty slow. It’s good for 55-grain bullets. By the way, legend has it that intentionally unstabilized bullets were the goal and reason for the 1-12. You know, the “tumbling bullets.” Hmm. 1-14 will work for those also, and that’s the Benchrest twist standard for 52-grain .224s. Didn’t happen. Barrels with 1-12 twist rates launched 55-grain mil-spec bullets in balanced harmony.

Sierra 90-grain MatchKing .22 Cal .224 diameter hollowpoint bullets and box

These are big bullets, but mostly they are long bullets — Sierra 90-grain MatchKings loose and loaded in a case. As the box denotes, these need a 1-6.5 twist to stabilize. I have also shot good groups with lighter bullets (77s and 80s) from that twist barrel, but I consider the 1-6.5 twist a specialty choice. I also consider the 90-grain bullet to be a moot choice because it’s tough to get the velocity/ballistic-coefficient balance to favor it. In other words, the 90 has a higher ballistic coefficient, but the faster velocity possible with an 80-grain bullet makes them equal downrange, when fired from a 20-inch barrel. Thus, there’s no effective advantage to the longer bullet that warrants buying a new barrel with the specialty twist rate.

For a good while, 1-9 was the standard twist rate for most non-mil-spec barrels. Another more modern standard is 1-8. This is good for the 75- and 77-grain bullets that are commonly available in factory loads. The 1-9 hits the limit of its utility with a tangent-profile 70-grain-vicinity bullet, a 69-grain Sierra MatchKing for instance.

Some say the 1-7 twist that became standard for the A2 version of the AR-15/M16 is too fast. It’s overly fast. It’s not too fast. I’ve shot way on too many clean targets at reduced distance NRA High Power Rifle events with my 1-in-7s. For those I use either 52-grain (100 yards) or 62-grain (200 yards) bullets.

In Sum

For any available factory loading I know of, which covers me in case any manufacturer does something different later on, a 1-8 twist is fine and dandy. I would advise shooters buy a barrel with 1-8 twist over one with 1-9 twist or slower just in case the shooter wants to try anything up to and including something like the 75-grain Hornady A-Max.

For .308 Win., I like 1-10. That will deal well with anything up to and including a 190-grain tangent profile, which means anything from Sierra.

Share your thoughts about barrel twist with us in the comment section.

SLRule

Glen Zediker has worked professionally with some of the greatest shooters on the planet, as well as leading industry insider rifle builders, manufacturers, and proven authorities on gunsmithing, barrel-making, parts design and manufacture, and handloading. And he does pretty well on his own: Glen is a card-carrying NRA High Master and earned that classification in NRA High Power Rifle using an AR-15 Service Rifle.For more, please visitZedikerPublishing.com or call 662-473-6107 (weekdays 9-4 CST). Write to P.O. Box 1497, Oxford MS 38655.

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Comments (46)

  • Mike Eshenroder

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    One of the better articles on twist rate. My experience with 1 in 12 224 55 grain is that you will get random wobblers with 55 grain. Some will produce ocassioal excellent 3 shot group where a 25 shot group will display the random wobbeling bullets. Unfortunantly the vast majority of shooters never shoot enough bullets into a target to see if the mythology they regurgitate about twist rates are truth.

    Reply

  • Sal

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    I know for a fact that 1/6 twist barrels do not “blow up bullets”. I have tested a couple 1/6 twist barrels for a manufacturer and this claim is flat out wrong. In fact the 10.5″ 1/6 twist barrel I tested shot better than a 10.3″ 1/7 twist barrel I had from DD. I have tested a 16″ barrel and the 10.5″ with bullet weights from 55 grain all the way up to 77 grain pills. I was actually shocked the the 55 grain bullets shot so well, in fact no different than a 1/7 or 1/8 accuracy wise, and with no signs of any bullet separation or stability issues. So your claim of “blowing up” any bullets. How many 1/6 twist barrels have you tested to back your finding?

    Reply

  • MrGman

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    oops, my mistake you are right. That’s where my brain was at the time. My apologies. G

    Reply

  • Secundius

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    @MrGman.

    The “MV” is Muzzle Velocity, not Mass x Velocity, sir…

    Reply

  • Secundius

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    @ MrGman.

    Your right it doesn’t impart Twist Rate to the bullet. What does do is give the bullet the 20,000-gravity “Swift-Kick-In-the-Ass” Velocity to travel down the barrel…

    Reply

  • Secundius

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    @ MrGman.

    I concede, you were and I was wrong. I apologize, my memory of Determining Bullet RPM’s was off, I forgot one of the components of the formula. MV (in fps) x (12/Twist Rate in inches) x 60 = Bullet rpm. I had to look it up in my DeskRef book. Sorry…

    Reply

    • MrGman

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      your on the road to recovery. mass (M) is not part or this particular equation. Mass times velocity yields momentum in foot pound seconds or kilogram meter seconds not velocity in feet per second. To determine RPM or RPS all we need is Velocity in FPS X 12 / the twist rate number of the barrel, X 60 if converter to RPM. Mass never enters that specific equation.

      Reply

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