AR-15: Why Twist Rate Really Doesn’t Matter For Stability Of Heavier Bullets
BY Herschel Smith
2 years, 8 months ago
Andy with practical accuracy sends me a note with a video he did as a challenge from a commenter here at TCJ.
First of all, it’s nice to be in the mix of thing where you can affect outcomes like testing of weapons systems. Second, his results are interesting and seem to me to put to bed the notion that a 1:9 twist rate can’t stabilize a 77 grain bullet. This is the same thing Steve Mayer at Rock River Arms told me several years ago. But it’s nice to see it tested.
By the way, nice shooting Andy. A 1 MOA group is always good in my book.
On March 7, 2022 at 11:55 pm, Georgiaboy61 said:
Each barrel is its own animal, and has characteristics unique to itself… which means that it must be tested and used in the field to see what it likes, does not like, and so on. In this age of CNC machines and precision machining and barrel-making, one might be surprised that this is still the case, but it is.
That means that a given 1:9 twist barrel might stabilize bullets over 75-grains well, whereas another which seems identical may not. Bear in mind, too, that it is not simply bullet weight which is crucial, but design, construction and length as well – all of which impact stability in flight.
A case in point is when the U.S. Army was considering the changes to the M-16/M-4 around the time M855 was being tested, back in the 1980s. Testing had determined that the 62-grain SS109/M855 did best, accuracy-wise, in a 1:9 RHT barrel. However, the 1:9 did not perform as well with the tracer round which weighed just two grains more, at 64-grains. The 64-grain projectile was longer, and of somewhat different design, enough to force the army to adopt the 1:7 instead, even though that rate was somewhat detrimental to accuracy with regular 55- and 62-grain FMJ.
Did the Army test a 1:8 rate? If so, how did it perform? If not, why not? if any of that work was done, it is now lost to history, at least as far as I am aware.
Moreover, a barrel which is borderline for a particular bullet may sometimes be coaxed into using it by upping the muzzle velocity. That’s why a bolt-gun with a 24″ 1:9 barrel may like a given projectile fine, but that 16-inch carbine 1:9 does not. Those additional inches of barrel and the subsequent higher MV matter.
Since BC values vary according to muzzle velocity, and thus spin rates also factor into it, achieving stability with a load in a carbine-length rifle or AR pistol may be more challenging due to lower MV values, and it gets tougher still as one approaches the trans-sonic and subsonic ranges. You may very well find that a tube which works great with a give load at ample supersonic velocity, may start to fall flat as MV drops. You will need a more-aggressive rate if these specialized loads are on your personal menu as a hunter or sportsman. In particular if you are using heavy-for-caliber projectiles.
For those who want to gt further into the somewhat arcane science of external ballistics, noted ballistician and aeronautical engineer Bryan Litz has a “twist rate” calculator up at his “Applied Ballistics” and Berger Bullets websites, one or the other, can’t recall which…. which allows you to calculate just how favorable or unfavorable a given load and twist rate combo really are.
On March 8, 2022 at 12:04 am, Herschel Smith said:
The army actually used a 1:8 twist rate for the M-4s on the proving grounds for the M855A1. Those are a slightly different animal than what he’s shooting.
https://www.captainsjournal.com/2017/02/19/ar-15-ammunition-and-barrel-twist-rate/
On March 8, 2022 at 2:44 am, Georgiaboy61 said:
@ Herschel Smith
Re: “The army actually used a 1:8 twist rate for the M-4s on the proving grounds for the M855A1. Those are a slightly different animal than what he’s shooting.”
Yes, of course that’s true. I was referring to the original M855 testing: Did the ordnance people even consider a 1:8 then?
The M855A1 issue certainly divided the Army and Marine Corps, didn’t it? It would be interesting to hear your son’s perspective on it.
I’d fall on the side of the Corps where that particular issue is concerned. The misgivings about the M855A1 were valid, in my opinion. I’m not claiming it isn’t a high-performance cartridge, but it looks like to me one that really tears up and accelerates the wear on the carbines themselves. But apart from that comment, I’m not up to speed on it enough to comment further.
On March 8, 2022 at 8:20 am, George said:
Years ago I shot with a Sierra Bullets employee who used nothing but 77 grain Match Kings. He shot them in a 1/9″ twist barrel. Very accurate, even out to 600 yards.
On March 8, 2022 at 8:36 am, Ohio Guy said:
All 5 of my ARs have 1/7 twist. 3 of them are 18″, the other two are 16″. @ 300yds, the 18″ barrels shoot 55, 62, and 75gr relatively the same at that distance on a 12″ plate. The 16″ barrels, however require a slight raise to lob it in. If only I had quick, convenient access to a 600yd range, I’m sure the doping would be significant. And Georgiaboy61 is correct on Brian Litz’ twist rate calculator as well as these guys: http://www.jbmballistics.com/ballistics/calculators/calculators.shtml
Someone else, I can remember who now, went thru the painstaking detail of different weight bullets thru different lengths of barrels, their velocity and drop. https://www.80lowerjig.com/80-lower-blog/the-ar-15-picking-the-best-barrel-length-twist-rate/
On March 8, 2022 at 8:40 am, J. Smith said:
I have/had numerous platforms ar to single shot in 223/5.56 i have1/12, 1/9, 1/8 and 1/7 twist barrels from 7″ to 26″ Light bullets 55g and especially 40g love 1/12 twist, 40 in a long barrel with 1/12 is amazing. Most of my 1/9 shoot 40-62 and some 75g great. My 1/8 wylde chambers are the best overall. All my 1/7 even my colt match are horribly inaccurate even with match black hills 75-77g, even worse with 55-62g Its a terrible twist. I got rid of all my 1/7 twist rifles, regardless of platform and barrel length. I know i probably just committed blasphemy against someones 1/7 religion. Im just sharing, dont start a debate or an argument im not responding.
On March 8, 2022 at 1:26 pm, Fred said:
Well, I like him; he took the challenge. Also appreciate the safety lesson. He’s fortunate the weapon was pointed at something he was willing to destroy.
On March 8, 2022 at 3:27 pm, Andy said:
Thank you Herschel for the nice comments and posting my video.
I agree with Georgiaboy61 that each barrel should be tested to see what bullet weight works best for the rifle. However, if a given 1:9 twist barrel does not shoot well with 77 or 75 grain bullets, I think we are too quick to blame the twist rate as the factor causing the problem.
If you are in fact getting keyholing in your target, I would say it’s the twist rate. Without keyholing, there are so many other factors that can produce less than acceptable performance (i.e. shooter error).
I have 4 Rock River arms ar15’s with a 1:9 twist rate and they all shoot 77 grain bullets very well. 3 of these rifles are featured in another longer video on my YouTube channel discussing twist rate and shooting at distance with them to demonstrate their accuracy. I purposely used 3 rifles to show this so someone would not be able to say I had a 1 off rifle that stabilized the projectiles.
The 4th rifle I regularly shoot out to 800 yards with 75 grain bullets. 77 work well too, but that barrel prefers 75 grain. I don’t have this particular rifle on video as it is an A2 and the limiting factor on hits with iron sights is not the twist rate, but shooter error. The A2 that I have on video on my channel shooting out to 1000 yards is a RRA 1:8 twist.
Moreover, the black hills 77 grain TMK box indicates these bullets should be used with 1:9 twist or faster. The Hornady 75 gr BTHP box indicates 1:9 twist or faster. I don’t think these manufacturers would indicate this on their box if these bullets were not stabilized by a 1:9 twist rifle.
On March 8, 2022 at 3:47 pm, Herschel Smith said:
@Andy,
I’ll say this for RRA rifles: they’re shooters. They are accurate and reliable. If you can’t hit with them, the fault is yours, not the gun.
On March 8, 2022 at 5:22 pm, Georgiaboy61 said:
@ Andy
Re: “Moreover, the black hills 77 grain TMK box indicates these bullets should be used with 1:9 twist or faster. The Hornady 75 gr BTHP box indicates 1:9 twist or faster. I don’t think these manufacturers would indicate this on their box if these bullets were not stabilized by a 1:9 twist rifle.”
Yep, RRA makes an excellent rifle. Their AR15 National Match model is a sweet-shooting rig, for much less money than competitors charge. Windham Weaponry also makes very accurate rifles; their 20-inch barrel “Governmental” model is a tack-driver.
Good research and presentation….
Here’s another factor to consider: Is your barrel actually twist rate shown on the outside of the barrel? An acquaintance of mine, a retired military officer and NRA-certified master reloader, shared this tip with some of his students. You don’t take the manufacturer at face value; you test the bore for its rate of twist and verify that it is what is claimed.
An ingenious method and easy to do at home: You’ll need a rotating coated cleaning rod of appropriate bore size, a screw-on loop and a few patches, enough to fit tightly into the barrel. After verifying the rifle is empty and safe, bring the bolt forward into battery and into firing position. Insert a few patches modestly lubricated enough to fit tightly but not too-loosely in the bore. When you insert the patches/loop, the rod should turn visibly. If you cannot see this, affix a small piece of colored tape to the rod. It should rotate as the rod is pushed down the bore.
When you reach the bolt face, the rod’s forward movement will be stopped. At this point, mark the rod right where it exits the muzzle, again using a small piece of tape. Slowly withdraw the cleaning rod/loop with patches, making sure that the rod spins continuously as it is withdrawn.
Here is the part where you need to pay attention: Withdraw the rod only enough to rotate the tape marker at the end of the rod one full revolution. Once it has reached this point, again mark the rod with tape and pull whatever length remains out of the barrel.
The rate of twist is measured as a ratio, such as 1:7 or 1:9 inches. This refers to the length of barrel needed to make one complete turn of the rifling. Now, using a measuring tape or ruler, measure the distance between your two pieces of tape affixed to the cleaning rod. It should correspond to the ratio listed on the outside of the barrel. If your barrel is marked “1:9,” then it should require nine inches of travel of the rod to turn 360 degrees.
On the other hand, the degree to which your measurement varies from the marked ratio tells you that the manufacturer either did not mark the rate correctly, or that the barrel is slightly out of spec.
If necessary for a clean and reproducible measurement, perform this procedure several times if needed.
It is generally worth going to this kind of trouble only when a high degree of precision and accuracy are required from a particular barrel or rifle. Or if you happen to enjoy such arcane technical matters as this…. this method is also useful as part of the procedure for diagnosing a barrel which isn’t performing as well as expected.
On March 8, 2022 at 5:32 pm, Andy said:
Georgiaboy61
Excellent point. I actually checked the barrel with this method 1 year ago after I posted the first test on video. I received an email from a person who doubted the rifle was 1:9 twist and suspected my barrel was 1:8.5. I measured it 3 times and it checked out right at 1:9.
On March 8, 2022 at 7:13 pm, X said:
The ability of a particular twist rate to stabilize a bullet is not simply about weight. It’s actually about bullet length, which is correlated with weight.
A 1/12 twist .223 barrel will stabilize the Speer 70-grain flat-based soft point with excellent accuracy. Conversely, it will never stabilize a 69 grain Sierra Match King boattail, which is longer despite being one grain lighter.
The ability of a 1/9 twist to stabilize a heavier bullet is based partly on how true the actual twist rate is, as some commenters have already pointed out, but also on which heavy bullet is being used. In my experience 1/9 rifles that have problems with 77 grain bullets will often stabilize the 75 grain Hornady HPBT, which is shorter. VLD bullets tend to be longer and may present issues, while Match King profile may have more success.
On March 8, 2022 at 8:20 pm, Andy said:
X
I agree with what you said and I will point out that I originally limited my twist rate videos to bullets that are loaded to magazine length. Many heavier VLD’s, as you know, are loaded beyond magazine length and are single feed with a sled. To that extent, I do not have experience shooting these extremely long bullets in a 1:9 twist. If I had to guess, shooters would start running into stability issues with these bullets that cannot be loaded to magazine length.
On March 9, 2022 at 3:12 am, Ohio Guy said:
@ georgiaboy61: Glad you brought up the “arcane” procedure of checking twist rate. As I am overly meticulous about the proper care and operation of my boomsticks, your comment inspired me to check all my barrels for proper twist to correspond with labeling. Thank you, sensei of the rifle! Your kung fu is strong!!!
On March 9, 2022 at 3:50 am, Georgiaboy61 said:
@ Ohio Guy
Thanks for the kind words, but I really can’t claim credit for the idea, because I did not think of it. My mentor, whose name I’ll withhold – deserves the credit. I do show up and pay attention, though, which helps!
@ Andy
I am not surprised that your barrels tested out right at 1:9. Most are marked accurately. A few are not, and it isn’t always as a result of negligence or poor workmanship, but maybe mixed of different production lots, that sort of thing. A batch of 1:9 barrels getting mixed in with the 1:7s and no one catching it before they were installed in rifles and carbines.
Length is important as a variable in considering stability and twist. At least that is what I was taught and what I have observed in my own experience. I’ve seen 75-grain Hornady A-Max shot successfully and accurately out of a 1:7 and a 1:8 but not out of a 1:9. I bet if you pushed it hard-enough and with enough barrel length it would work. But no way to know save trying it out, right?
Single-loading? Yeah, that’s what you do if you want to run them in a standard magazine length AR15. That’s why .224 Valkyrie was invented, to allow heavy- and long-for-caliber bullets to be fired out of ARs. At least that is my understanding. Thicker but shorter case to allow longer projectiles room to fit in a standard short-action chamber.
But getting back to stability in a 1:9, it would make a fascinating experiment to test some 75-grain or 77-grain loads at various distances and see if the marginal twist rate shots lose stability and start to keyhole sooner than the ones fired from more aggressive twist rates. Rifle bullets tend to be tail-heavy, which means that the proper nose-first attitude is gained via high rpms and the phenomenon of spin-stabilization. What any Physics 101 student learns as the “right hand rule,” in other words.