Is 1:9 Twist Enough To Stabilize 77 Grain Matchking Bullets?
BY Herschel Smith
3 years, 8 months ago
In previous posts we’ve discussed barrel twist and stabilization of heavier 5.56mm rounds. I’d say that 1:9 twist is just fine for 77 grain bullets based on his results. Then again, Rock River Arms makes good machines.
On February 25, 2021 at 1:16 am, Georgiaboy61 said:
For those interested, Applied Ballistics – the fine company run by aeronautical engineer and competitive shooter Bryan Litz – has a twist-rate calculator up at their website. Litz is also chief ballistician for Berger Bullets, and a highly-respected authority on bullet design and internal and external ballistics.
Every barrel is different, and so empirical testing is indicated to discover precisely what each particular tube “likes” and “dislikes.” Bullet weight isn’t the only relevant criterion; length and the shape of the projectile matter also, as do the material(s) from which it is constructed. Naturally, the characteristics of the barrel matter also.
Hand-loaders know that sometimes in a borderline stability-instability situation, high muzzle velocity will push a bullet from unstable to stable, and allow it to perform well. Even then, testing is indicated, because every barrel is different.
A case in point: According to theory, a 1:7 twist barrel ought not to shoot 55-grain or lighter bullets as well as a slower 1:9 or 1:12 twist, but I have used 1:7 barrels that shoot superb groups with low SDs and ES values, using just such lighter bullets. The theory suggests that this should not be likely, perhaps not even possible, but there are the results.
Modern metallurgy and manufacturing techniques now deliver more-consistency and quality on a sustained basis than ever before when it comes to factory mass-produced barrels, but even then, some outliers slip through. Which is why field testing is required if you really want to get a handle on that particular firearm.
In the late 1970s and early 1980s, when NATO decided to transition to a different 5.56×45 mm bullet as STANAG for the cartridge, the U.S. Army undertook testing of the proposed new 62-grain FMJ/Ball round with penetrator, commonly called “green-tip” – and also the proposed new tracer, which weighed 64-grains. These they tested in a variety of lengths of barrel, twist rates and other conditions.
It was found that the best all-purpose twist rate for M193 55-grain Ball, as well as for the new 62-grain FMJ bullet, was a 1:9 twist. In comparison to the faster 1:7 twist, it shot tighter and better groups at the distances tested. However, the 64-grain tracer was another story. Because of its longer body and its weight, the tracer round would not adequately stabilize in a 1:9 twist barrel.
Needing the ability to fire tracers accurately was – in the end – the reason why a 1:7 twist rate was chosen for the Colt military contract M16s and M4s then coming off the lines. I cannot recall if a 1:8 twist was tested or not; it would seem to offer a nice compromise between the 1:7 and 1:9.
Another factor in how well a given bullet shoots in a given firearm is the chamber design. Military-specification weapons, such as the M16 and M4, since they offer select-fire capability, need to have more-generous chamber dimensions as an aid to reliable function when the mechanism is dirty, hot and under real-world operational stress.
Consequently, mil-spec small arms and crew served weapons tend to have longer free-bore dimensions and looser chambers over all. Hence, longer “jump” for the bullet to engage the lands. Some precision and accuracy is sacrificed by these characteristics, but that’s an acceptable trade-off under the circumstances. If the grunt’s weapon doesn’t go “bang” when he pulls the boom-switch, all of that match-grade accuracy and what-not is for naught.
What does this have to do with stabilization and twist rate? If the bullet does not seat squarely in the throat of the barrel and is inconsistent in its engagement of the rifling, it will be handicapped by the instability thus introduced into its flight. Even expensive hand-turned solid metal bullets are not 100% concentric, and despite amazing advances in bullet materials and design in recent years, neither are factory projectiles. This means that virtually all bullets fired will yaw about a central axis of rotation. The greater the yaw, the greater the propensity for inconsistency in flight.
Tighter chamber dimensions, such as those in a Wylde-style chamber, will lend themselves to better accuracy, stability and performance. They need to be kept cleaner than looser chambers to function optimally, which is one of their trade-offs.
Like a top spun at very high speed, it takes a period of time for a just-fired bullet to sort itself out in terms of consistency of rotation. During this somewhat inconsistent phase, accuracy may be mediocre or poor, but once stability is achieved, or in layman’s terms – the bullet has “gone to sleep” – it is likely to be much more-consistent and accurate.
Barrel length is also important. Even if the proper twist is selected, if sufficient revolutions-per-minute of spin are not imparted to the bullet, it will not be stable in flight, or will not be stable for as long as it might otherwise be. I’ll leave the precise calculations to the mathematicians in the house, but once spin degrades enough and the bullet slows down its rate of rotation sufficiently, it will become dynamically unstable and want to “swap ends” and resort to putting its heavier aft portion ahead of its lighter tip. Most bullet designs are tail-heavy anyway; the only thing that keeps them in the proper nose-first attitude is spin-stabilization.
Barrel length brings us back to the idea of using high MV to stabilize a borderline unstable projectile. Push that .224-caliber 77-grain Match King BTHP fast-enough out of a fairly-long 1:9 twist barrel, and you’ll probably have stability, at least for a while. It’s easy to diagnose, since unstable projectiles “keyhole” on the target paper or backboard.
Herschel, getting back to your original question about the 77-grain Black Hills TMK in a 1:9 barrel, if the bullet stabilizes, it would be interested to know how long it would remain that way, versus one shot out of a 1:8 or 1:7. Maybe Black Hills has some data on that…..
On February 25, 2021 at 2:26 pm, TRX said:
Rock River was one of the companies that backed anti-gun legislation in Illinois with SB-1657. They tried to doubletalk their way out of the bad press and made themselves look foolish as well as duplicitous.
It’s unfortunate that they’re still in business, and I certainly have no interest in anything they make or say.
On February 25, 2021 at 4:41 pm, Silence DoGood said:
The force needed to stabilize a bullet changes at the square of the density of the medium it is passing through. Air gets denser as it gets colder and as that happens bullets get harder to stabilize.
The guy in the video is wearing short sleeves so let’s stipulate it was 70°F when that was recorded. Air at the same pressure is not quite 2% more dense at 60°F than at 70°F and not quite 2% squared comes to (2^2=) not quite 4% more gyroscopic stability needed to keep the bullet stable. Because the air is cooler.
So there’s nothing in this “test” that would indicate whether the bullet would remain stable if the air temperature was significantly cooler than when recorded.
As to barrel length, according to Bryan Litz’s Applied Ballistics for Long Range Shooting, 1st Edn, Ch 10, Bullet Stability, p.150, where stability is concerned, the velocity that comes from a longer barrel has negligible effect on bullet stability because with that added velocity also comes a greater overturning aerodynamic torque acting on the bullet’s Center of Pressure. So the increased overturning moment takes back near as makes no difference all of the stability gained through the longer barrel’s higher rotational velocity. A longer barrel is NOT an effective means of improving bullet stability. So sayeth Bryan Litz.
On February 25, 2021 at 4:47 pm, Silence DoGood said:
I should add that according to independent testing done by Bryan Litz, bullet/twist combinations resulting in an Sg of less than 1.5 still nutate enough to diminish the bullet’s effective ballistic coefficient (the Army’s standard minimum Sg is only 1.4). So just because a bullet isn’t tumbling doesn’t mean that you’re getting the best possible benefit from its BC. The bullet will shoot all the flatter and remain stable further downrange if you select a particular bullet that will produce an Sg of at least 1.5 from that particular barrel.
On February 25, 2021 at 4:52 pm, Herschel Smith said:
Nothing in this post or the video stated that barrel length has anything to do with it. The issue was barrel twist.
Next, if I’m not mistaken what you said runs contrary to what the Army found in their testing, to wit, one of the correctives for the shorter barrel lengths they were fielding was to change the twist.
567 yards away is a LONG way for this round. To me it still looks like the bullet is well stabilized with the 1:9 twist. What affect temperature would have on it would have to be tested. I’d have to see data.
Finally, as for the effectiveness of the round, that has more to do with the bullet. If you want a longer range round, change to 6.5 Creedmoor.
On February 25, 2021 at 5:48 pm, Andy said:
Hershel, I am the one in the video and I believe the air temp that day was around 80 degrees.
I have not tested in weather colder than 60’s for this round and twist rate. At that temperature, the results were the same for me.
I have shot this bullet out to 1000 yards with a 1:8 twist with good results. Shooting in 80 degrees Plus and 1000 feet above sea level (Indiana).
I have not shot it out of a 1:9 any further than 567 yards, but I have no reason to believe that it won’t stabilize. I will test this spring with the 16 inch I used in the video and a 20 inch 1:9 twist I used in another video.
I have shot the Black Hills 77 grain in 2 other videos on my channel with the above mentioned 20 inch 1:9 twist and I have been able to get sub Moa (barely lol) performance so I believe I will be able to reach out to 1000 yards (at least that’s the goal) this Summer with this combination.
https://youtu.be/2y3skScIZ_M
On February 25, 2021 at 6:06 pm, Herschel Smith said:
Thanks for visiting Andy. Keep me posted on this.
On February 26, 2021 at 11:51 am, John said:
Ruger chose 1:9 twist for its revamped Mini 14 back around 2005. Smart boys at Ruger.
On February 27, 2021 at 10:26 am, NOG said:
John, I don’t think it matters. I owned a early 180 series 1/9 Mini. It would not group, only pattern. No matter what bullet I tried. I sold it for half what I paid for it. Married a great woman years later. Not a shooter. I taught her to shoot with a 1022. I then suggested she needed a centerfire rifle. Tried to teach her on my AR. She went to town for groceries and came home with……a Mini 14. Groan, but I said nothing. She liked the Mini because it “felt like my 1022”. It was a early 187 series with a 1/7 twist. Did not shoot well. I bedded the action, opened the gas tube one or two sizes (been a while- old memory) and got a barrel strut. Made it heavier but… from the bench with a 3/9 scope I now got 1.5 inch groups (100 yards) with Black Hills 68gr OTM. I got 1.8 inch groups with some old Privi M193, not materially different in my book. Bottom line I think barrel quality and proper build technique counts way more than twist rate. Today she has a AR pistol. Older people need different equipment. She still misses that Mini.
On February 27, 2021 at 12:08 pm, John said:
“Revamped around 2005.” Times, and Mini 14’s have changed.
On February 27, 2021 at 1:55 pm, RHT447 said:
Brief bona fides–Graduated Small Arms Repair School at Aberdeen Proving Grounds in ’74. Started handloading in ’77. Got my NRA Highpower Master card in ’90.
All I can add to the above comments is that you won’t know until you shoot it. Here are a couple of anecdotes–
1) Rifle was an AR H-Bar A2 that I built in the early 90’s for Highpower competition. 20 inch barrel, 1:9 twist. Later in June of 2000, I wanted to try out some Hornady 75 gr A-max bullets (that I molly coated myself). Time was a bit tight then, and the only chance I had to shoot was an upcoming match. So I loaded up 10 rounds for the slow fire prone stage–24.5 gr of Varget, bullets seated long to .010″ off the lands.
The match was shot at 200 yards with pit targets. The slow fire prone target was the MR-52 (reduced from 600 Yard MR-1). All shots managed to hit inside the 5 ring. Vas not group. Vas pattern. Other shooters that day suggested that Reloader 17 might be a better powder, but I did not experiment further.
2) Moving on to ’09. Rifle is a factory stock Winchester M70 with a 26″ SS fluted heavy barrel, 1:9 twist. Load is the same as above, except 25 gr. of Varget. Shot off the bench at paper target at 250 yards. Center to center for 5 rounds, 3/8 inch.
On February 27, 2021 at 3:26 pm, Herschel Smith said:
@RHT447,
I quite agree. You’ll never find an engineer who says, “Nah, I don’t need any more real world data to confirm my models. I have enough.”
On February 27, 2021 at 9:17 pm, Andy said:
Herschel, lots of good comments here. I did want to add to the previous comments and state the reason I made the video was in response to the “blanket” comments I hear in person, online or read online that go something like this. “I bought (or will buy) a 1:7 twist so I can stabilize heavier bullets because a 1:9 can’t stabilize that bullet.” I hear this quite a bit and it runs contrary to my personal shooting experience.
I believe these commentators are confusing stability and precision. (I am interchanging precision and accuracy here even though I know they have separate meanings.) I believe they think buy a 1:7, put heavy bullets in, and get an accurate/precise gun.
A bullet can be completely stable, but not precise as other comments have said. A simple example is of a football quarterback throwing a perfectly tight spiral 10 yards behind the receiver. Completely stable pass, but not accurate.
A more recent example for me is that I completed my first Ar-15 build. I used a criterion chrome lined 1:8 twist barrel. According to “blanket rules” this barrel should stabilize (i.e. shoot precise groups) with 77 gr. TMK. Initial groups printed a lousy 2.5 to 3 inches @ 100 yards. However, initial testing with Frontier 68 gr BTHP printed outstanding .6 groups at 100 yards.
I should also probably follow up in another video to clarify that just because you get a 1:9 twist doesn’t mean your gun will shoot precise groups with 77 gr TMK. The point of the video is it can be done. I agree with the comments that real world testing needs to be done on your specific barrel.
My personal experience with Rock River barrels is that they shoot 77 gr. TMk very well with factory loads. (I don’t reload yet so I don’t get to fine tune loads). I am currently building another Ar-15 with a RRA barrel so I will be doing more testing.
As I mentioned in a previous comment, I will be testing at longer ranges this Spring and testing with a couple criterion barrels as well as RRA barrels.