AR-15 Ammunition And Barrel Twist Rate

The Captain’s Journal – by Herschel Smith

There are a lot of articles and discussion forum threads on barrel twist rate for AR-15s.  So why am I writing one?  Well, some of the information on the web is very wrong.  Additionally, this closes out comment threads we’ve had here touching on this topic, EMail exchanges I’ve had with readers, and personal conversations I’ve had with shooters and friends about this subject.  It’s natural to put this down in case anyone else can benefit from the information.  Or you may not benefit at all.  That’s up to you.  

This is a discussion about 5.56mm ammunition and barrel twist rates (and later, about the shooter and ammunition quality).  If you wish to debate the effectiveness of the 5.56mm round generally, or wish to disparage the choice of the Eugene Stoner system, I’m sure there are forums for you.  This is not it.

In the real world, ammunition isn’t concentric, and even if it is almost precisely concentric, pour density can be slightly different throughout the ball, and voids can develop.  This causes gyroscopic stability problems with bullets, even in the best manufactured ammunition. But much ammunition would not be considered the “best manufactured ammunition.”  Ammunition will only be as good as the QA under which it was made.

When center of gravity is off-axis it can cause bullet lateral throwoff, yaw and a host of other problems with bullet trajectory.  In order to overcome these problems, rifling twist achieves this gyroscopic stability for the bullet, thus negating the effects of the manufacturing process (at least in part).

Overstabilization can occur with a barrel twist rate that is too high.  There are incorrect commentaries out there on this subject.  This writer explains that higher twist rate is virtually always better.

Conventional wisdom taught us that slower twist rates wouldn’t properly-stabilize a bullet, causing it to yaw. On the other hand, faster rates could over-stabilize lighter bullets, causing similar problems. This is correct in theory—however, modern ballisticians have pretty much de-bunked the over-stabilization theory as a practical matter. All things being equal, it is better to have too much twist than not enough.

While his statement is a bit imprecise, there is something very precise about it.  It is precisely wrong.  Yet there are much cleaner and simpler explanations of why high twist rate is not always good.  One commenter at this discussion thread summed it up well.

You can certainly overstablilze (sic) a bullet if you spin it so fast it doesn’t nose over at the top of its trajectory … Best thing to do is not spin bullets any faster than what’s needed for best accuracy.

Correct.  If a bullet is overstabilized, it tends to stay pointed along its axis of rotation, even on the final (downward) part of its trajectory.  This can cause keyholing, odd aerodynamic effects (flying sideways through the air) and even bullets to wildly spin off trajectory.

Above it was noted that displaced CoG can cause gyroscopic stability problems, including “lateral throw-off.”  This figure is given to us by Paul Weinacht in his paper for the U.S. Army (Army Research Laboratory, ARL-TR-3015) entitled Prediction of Projectile Performance, Stability, and Free-Flight Motion Using Computational Fluid Dynamics (Figure 9).

Or if you wish to visualize what this might look like in 3D … Dean and LaFontaine, Small Caliber Lethality: 5.56mm Performance in Close Quarters Battle.

Bullets from rifled barrels eventually achieve stability by yawing back and forth, while undergoing a larger revolution about the major axis of the trajectory.  So quite obviously, it’s necessary to spin the bullet, and to spin it enough to give it stability, while protecting the need to nose over on the final part of its trajectory.  Getting this twist rate and spin right has been a matter of much testing, internet fights, and lot of engineering study and heavy spending by the taxpayers.  I know that my guns perform well, and so I decided to contact my manufacturer for his opinion on the matter.

In the interest of full disclosure, I have two Rock River Arms rifles, one Elite CAR A4 with a 16″ Barrel, twist 1:9, Quad Rail, and another competition gun with a muzzle brake and 18″ SS barrel with a twist rate = 1:8.  I have recommended RRA rifles to my readers before, but there are many good guns on the market.  Your probably have one.  I sent a list of three questions to RRA, and Steve gave me these responses (the question isn’t included because it wasn’t forwarded back to me, but it’s apparent what I asked except for the first question, which was basically does RRA warranty their 1 MOA for both M193 and M855.  This is Steve’s response.

Herschel,

Thanks for your questions.  I’m going to take them in reverse order.

3.  1:9 is adequate for many, but not all rounds typically used in an AR platform.  Between .223 Remington and 5.56mm NATO, there are rounds from 45 to 90 grains (that I am familiar with) and I know of, but have never shot, lighter and heavier rounds.  No single twist is going to handle all of them.  1:9 is adequate for a sizable number of them, however…including the two most commonly available, in bulk and at reasonable prices…55gr FMJ (M193)and 62/63gr FMJ (M855).  It is not ideal for rounds lighter than 50gr nor those over 68 or 69 grains, which is why there are other twist rates commonly available…including from RRA. We offer a 1:12 24” bull barrel for our Varmint hunters who prefer to use the lighter bullets for prairie dogs and other targets, and both 1:7 and 1:8 barrels in a variety of configurations for those who want to shoot heavier bullets…up to and including the newer 77gr loads and 80gr VLDs.  We’ve also run custom twists for a limited number of contracted purchases.

2.  Yes.  1:9 does well with both M193 and M855.  Different barrels perform differently, but 1:9 generally stabilizes both weight/length bullets fairly well,  It neither over nor under spins either and does not produce key holing.

1.  The hardest question to answer.  Neither M193 nor M855 are notoriously accurate rounds.  They meet military, not match, requirements.  Our accuracy claims are the rifle’s capability…but the shooter and ammo have to do their parts.  There are loads that are commercially available and claimed to be “M193” and “M855” equivalents that clearly aren’t, and they aren’t  capable of  ”minute of bad guy” at 100 yards, let alone the .75 to 1.5 MOA claims that we make for our different rifles.  That is no reflection on our rifles or barrels, or the shooters…unfortunately there is some real crappy ammo on the market today, which will not perform well out of any barrel, of any twist rate.

Thanks.

Steve/RRA

This is a good response, but let’s not stop here.  While perhaps not recalled by some, American Rifleman has given us a fairly comprehensive look at 5.56mm ammunition and barrel twist rates in an article entitled Testing The Army’s M855A1 Standard Ball Cartridge.  It is rich with history on how the Army fielded the M855A1.  Ignore the issue of the M855 versus the M855A1 for a moment and consider the background.

Accuracy cannot be assessed without addressing the rifle barrels’ twist-rates. In the early 1980s the M855’s 62-grain bullet was developed for the M249 Squad Automatic Weapon (SAW). For purposes of interoperability, the same load was adopted as the M16A2 rifle’s standard ball as well. A February 1986 U.S. Army study noted that the M855’s bullet required a “1:9 twist [which] would be more appropriate for the M16A2 rifle, improving accuracy and reliability.” Multiple studies confirmed the 1:9-inch twist requirement.

But then a problem arose. The U.S. military’s standard M856 5.56 mm tracer round was longer, heavier (63.7 grains) and slower than the M855 ball, and simply would not stabilize with a 1:9-inch twist barrel. Thus, despite it doubling M855 group sizes, the M16A2 (and later, the M4) specified a 1:7-inch rate-of-twist barrel to stabilize the tracer round. It remains so to this day. Therefore, M855A1 was test-fired with both 1:7- and 1:9-inch twist barrels, and it was verified that this new cartridge is consistently more accurate in the latter barrels-as was its predecessor.

Don’t slip past these paragraphs, because they explain why “Milspec” is 1:7.  It isn’t because 1:7 shoots M193 or M855 more accurately.  It’s because of the weight of tracer rounds.  As we’ve discussed before, the term Milspec doesn’t mean better, or worse, or anything at all except that it precisely meets the specifications outlined in the purchase order(s), excepting whatever variance notifications they might make on a given batch of guns.

The M855A1’s developers have described it as yielding “match-like” accuracy, which most rifle shooters would define as one minute-of-angle (m.o.a.), or groups measuring no more than 1 inch at 100 yards. While the new ammunition has proved more accurate than the green-tipped load it replaced, testing did not yield match-like accuracy, especially in the standard 1:7-inch twist-rate found in today’s M4s and M16s. At 100 yards, the best group with a 1:7-inch barrel was 1.62 inches (1.6 m.o.a.). At 300 yards. it similarly fired 1.6 m.o.a. (4.9 inches) and widened to 1.8 m.o.a. (7.5 inches) at 400 yards. At these same distances, firing the M855A1 through a 1:9-inch twist barrel reduced group sizes by approximately half.

The tests demonstrated that 1:9 twist produced better accuracy, approximately twice as accurate.  Now take note what the testers found with the newer M855A1 regarding repeatability.

On average, the new ammunition produced one flyer in roughly each five rounds, which, it can be argued, exaggerated the group sizes. Since the Army announced that, “On average, 95 percent of the [M855A1] rounds will hit an 8×8-inch target at 600 meters,” each group’s most errant bullet impact was discarded and group sizes recalculated. Statistically they improved, but not enough to place 95 percent of rounds so close at 600 meters, at least when using the standard 1:7-inch barrel-which may explain why accuracy was less than expected.

There is one “flyer” in every five rounds.  This seems to me to be a significant problem with this ammunition combined with the barrel twist, and the commenters don’t seem to like it very much either.  Finally, this.

When U.S. Army shooters twice fired public demonstrations of the new round, they did not employ standard 1:7-inch twist M16A2s or M4s, but accurized, match-grade, stainless-barreled rifles from the Army Marksmanship Unit (AMU). I contacted the AMU and learned that these rifles did not have standard-issue 1:7-inch barrels, but most likely 1:8-inch twist, which probably accounts for their “match-like” accuracy.

Isn’t that rich?  The Army made claims of “match-like accuracy,” and proved the rounds shooting out of different barrels than are deployed with Soldiers, using 1:8 twist, not 1:7 twist.

The American Rifleman article goes on to discuss in some detail the performance of the M855A1 with slim-profiled targets like malnourished tribal fighters in Afghanistan (so-called “ice picking” the target without fragmentation), performance at barrier penetration (concluding that it is better than its predecessor), and its lethality once it does penetrate barriers.  I recommend this reading to you.  It’s well worth the time.

So to summarize what we know, remember some basic things.  First, the bullet has to be spun to give it gyroscopic stability.  This spin needs to match the bullet (including mass and length), and care must be taken not to over-stabilize the bullet.  If you shoot typical .223 ammunition (55 gr.), or M193 or M855, a twist rate of 1:9 is probably just about ideal.  You’ll probably lose some accuracy with a higher twist rate.

This loss of accuracy is likely not significant for a lot of shooters.  If you shoot much heavier ammunition (and there is a lot on the market), you probably need to consider a twist rate of 1:8.  Finally, none of this matches the value of good ammunition or good shooting.

That’s the good news.  Most guns can outperform the shooter, and I know that’s the case with me.  I’m a decent shooter.  Not great, but decent.  I’ve taken my Tikka T3 .270 bolt action rifle and literally put rounds through the same hole at 100 yards (with slightly more tearing of the same hole in the paper).  On the other hand, this is with a good scope, no wind, a cool and comfortable day, all day to work my craft and thus no time pressure, no one else to be concerned about, lots of coffee to wake up, and a full belly.

But if I had kept records, it wouldn’t have happened again exactly like that since, theoretically, even with perfect ammunition, considering barrel harmonics and that physical processes like this are a heuristic phenomenon, if I had continued to log my shots this way, it would have doubtless shown a standard distribution (distance between each shot and mean).

But regardless of the details, you’ve done it before.  Control breathing … get good sight picture … back out of the shot if you’re not mentally right … know where your trigger breaks … and so on.  You know the drill, since you’ve done it many times.  It’s perhaps the purest pleasure a shooter can have.

Now throw in simple annoyances like a whining partner at the range, losing daylight and time pressures, hunger, and any of the other 100 possible nuisances that can sap your accuracy.  Then your accuracy goes to hell, doesn’t it?  Now, combine that with wearing heavy gear and being shot at, and I’m sure it diminishes your control over your weapon.  Thankfully, I only have the experiences of my former Marine son conveyed to me.

The good part of this is that regardless of your barrel twist rate, if your AR-15 is reliable, even if it’s not top of the line, it can probably outperform you.  That means getting better isn’t a matter of getting a new rifle or barrel with a different twist.  It means practicing with your rifle, sometimes under duress.  It also means buying good ammunition.  Steve at RRA is right.  The shooter and ammo have to do their part.  I object to cheap ammunition just like I object to cheap engine oil.  I’m trying to develop the discipline at the store or online to buy better ammunition.

Right, I’ve got it.  I feel your objection.  Good ammunition (e.g., Hornady $2 per round .270 for my Tikka) hurts.  This is my wealth, and it’s hard to part ways with it since it’s hard to earn it.  But using bad ammunition at the range makes it hard to impossible to assess your practice.  Use of my value pack Federal .223 at the range means that my accuracy is irrelevant if I’m using the same reticle holdovers I would for 5.56mm since the muzzle velocity is different (and very slightly lower than the 5.56mm).  You’ve got the picture.

The best way to get better accuracy is probably not to get a better gun.  It’s to practice with the one you’ve got.

Here is a related video I found interesting on gyroscopic stability.  He’s wrong about the math being incomprehensible, but it is rather difficult if you’re involved with partial differentials or worse, the Navier-Stokes equations in CFD.  You need some specialized training in mathematics in order to tackle that.  You don’t have to know any of that in order to understand the basics of shooting.

This discussion probably won’t end the debate on barrel twist rate, and it certainly won’t end the fight between the Army and Marine Corps (who doesn’t want to deploy the M855A1).  But I hope it was helpful to you.

http://www.captainsjournal.com/2017/02/19/ar-15-ammunition-and-barrel-twist-rate/