Twist Rate: Common Misconceptions about Twist and Stabilization
Understanding Twist: Bullet Stabilization
by Sierra Bullets Ballistic Technician Paul Box for Sierra Bullets Blog.
Based on the questions we get on a daily basis on our 800 (Customer Support) line, twist is one of the most misunderstood subjects in the gun field. So let’s look deeper into this mystery and get a better understanding of what twist really means.
When you see the term 1:14″ (1-14) or 1:9″ twist, just exactly what does this mean? A rifle having a 1:14″ twist means the bullet will rotate one complete revolution every fourteen inches of the barrel. Naturally a 1:9″ turns one time every nine inches that it travels down the barrel. Now, here’s something that some people have trouble with. I’ve had calls from shooters thinking that a 1:14″ twist was faster than a 1:9″ because the number was higher with the 1:14″. The easiest way to remember this is the higher the number, the slower the twist rate is.
Now, the biggest misconception is that if a shooter has a .223 with a 1:8″ twist, his rifle won’t stabilize a 55gr bullet or anything lighter. So let’s look at what is required. The longer a bullet is for its diameter, the faster the twist has to be to stabilize it. In the case of the .223 with a 1:8″ twist, this was designed to stabilize 80gr bullets in this diameter. In truth the opposite is true. A 1:8″ will spin a 55gr faster than what is required in order to stabilize that length of bullet. If you have a bullet with good concentricity in its jacket, over-spinning it will not [normally] hurt its accuracy potential. [Editor’s Note: In addition, the faster twist rate will not, normally, decrease velocity significantly. That’s been confirmed by testing done by Bryan Litz’s Applied Ballistics Labs. There may be some minor speed loss.]
Many barrel-makers mark the twist rate and bore dimensions on their barrel blanks.
Think of it like tires on your truck. If you have a new set of tires put on your truck, and they balance them proper at the tire shop, you can drive down a street in town at 35 MPH and they spin perfect. You can get out on the highway and drive 65 MPH and they still spin perfect. A bullet acts the same way.
Once I loaded some 35gr HP bullets in a 22-250 Ackley with a 1:8″ twist. After putting three shots down range, the average velocity was 4584 FPS with an RPM level of 412,560. The group measured .750″ at 100 yards. This is a clear example that it is hard to over-stabilize a good bullet.
Twist-rate illustration by Erik Dahlberg courtesy FireArmsID.com. Krieger barrel photo courtesy GS Arizona.Similar Posts:
- Common Misconceptions about Twist Rate and Stabilization
- Twist Rate and Stability — Correcting Common Misconceptions
- Barrel Twist Rate and Bullet Stability — What You Need to Know
- Berger Bullets Twist Rate Stability Calculator
- Optimize Bullet RPM with Berger Twist Rate Stability Calculator
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Tags: .223 Remington, Barrel, Bore Diagram, Krieger, Sierra Bullets, Twist Rate
Have gone thru many a 1-7 1,7.7 and 1-8 twist .223 barrels over the years and all have shot 52`s, 53`s and 55`s very well.
“it is hard to over-stabilize a good bullet”
I would add: As long as the bullet can handle the rpm.
I’m not really arguing with the article.(just adding a caveat) There are bullets that are good at “X” rpm that will not even make to the target at “X-plus” rpm. Concenticity is not the only factor. A bullet that shoots great at a lower rpm, but that has a thin jacket and soft core, may not handle a higher rpm without disintegration.
My 2 cents worth:
The issue of “over-stabilisation”.
Can’t happen? Maybe.
The supposed point of spinning a bullet is to keep it CLOSE to a standard / predictable trajectory.
ALL bullets (and “big stuff”) display some degree of precession; spiralling AROUND the nominal trajectory. The smaller the precession, the smaller the “error” in target.
HOWEVER, what happens if the bullet is spun “too fast”?
A bullet spun according to the “rules” behaves in an interesting manner as it trundles down-range. Firstly, it exits the muzzle more-or-less aligned with the direction in which the last 5 to 10 calobres of the barrel were pointing at the time. Different load, different bedding etc. will result in different barrel vibration / whip and thus a different line of departure.
ANYHOO….the bullet is “suitably” stabilised to try to remain on this “line of departure”.
However, there are other players in the game: Gravity and Drag.
Dealing with Drag first: from the instant the bullet leaves the muzzle, it is no longer accelerating, but is pushing hard through the very air we breathe. With a supersonic rifle bullet, the “drag” or “air-resistance” is in the form of “shock cones” that form, primarily at the pointy end.
As this resistance has its wicked way with the bullet, these shock-cones start to creep rearwards along the bullet. NOTE; spin rate is essentially unaffected by this resistance. Thus, the ratio of “spin rate” to velocity INCREASES as drag acts.
The “tail” of the bullet at these velocities has very little influence at this stage.
When the velocity has dropped to the “trans-sonic” region, it gets VERY interesting.The shock cones “fall off” and “base turbulence “and “laminar-flow” resistance take over, always acting to slow the bullet. But, at this juncture, we are not concerned, by sub-sonic bullets.
IMPORTANTLY, the rotational velocity is largely unaffected by drag and is still trying to hold the bullet in the same line.
Now, we all know that the trajectory is, more or less, a parabola. If the muzzle is elevated to say, ten degrees to achieve another glorious “X” shot, the stabilised bullet will tend to STAY aligned on that ten degrees because that is the entire point of spinning the thing in the first place; if it goes wandering off elsewhere, the whole process is pointless.
The catch is that this is (mostly) the “REAL” world, in which we mere mortals are subject to the Laws of Nature in general and Physics in particular.
Assume a PERFECTLY made bullet made from perfect materials fired from a perfect barrel with a perfect crown. Said bullet would have its longitudinal centre of mass PERFECTLY aligned with its aerodynamic centre-line, which in turn would be PERFECTLY centred in that PERFECT bore. Good luck with that…….
What happens to the rest of us is this:
The bullet is fired up the barrel, mostly centred in the bore. As it leaves the crown, it gets interesting.
This is the first time the base of the bullet really starts to have an influence. As the base of this “perfect” bullet exits the muzzle, the high-pressure propellant gases “leak” out between the bullet and the crown. If the (in this case, flat) base of the bullet is not PERFECTLY aligned with our “perfect” crown at this critical time, the gas ‘eruption” will be anything but even around the circumference. This may result in very short-lived but quite violent force being unevenly applied to the rear of the bullet, making it “wobble”. Because the bullet is spinning at “ludicrous speed”, this wobble will be “slight” and the projectile will, of course tend to depart on its intended trajectory.
BUT, the bullet is now essentially “spiralling” about its own centre of mass. The “gyroscopic” forces are attempting to pull it back into line, whilst the “interesting” supersonic aerodynamic forces are also doing something similar. This “spiralling” is “precession” and is, apart from “operator error”, the primary factor in “groups” from otherwise insanely precise equipment.
There’s more. The bullet “wants” to continue on the line of departure from the muzzle, but gravity is dragging it down and drag constantly slows it down. At some point, the bullet will be more-or-less aligned in that original “line of departure”, BUT, its trajectory will be on a somewhat different angle. Something has to give.
The forces acting on the bullet will “adjust” its alignment.
If this happens in tiny increments and fairly often, the deviation from trajectory should be very small and groups will be “nice”.
HOWEVER, if the bullet is OVERSTABLISED, it will try to stay on its original alignment until WAY out of whack (relatively speaking). At some point shortly thereafter, aerodynamics will momentarily triumph and will snap the bullet back to be more-or-less aligned with the trajectory. It is this “more-or-less” part that gives us the odd situation of group size not being “linearly predictable” as range increases. In “extreme” cases, it also partly explains the “funny-shaped” bullet holes at some ranges.
Well, that is my understanding of things, anyway.
“In the case of the .223 with a 1:8″ twist, this was designed to stabilize 80gr bullets in this diameter. In truth the opposite is true.”
This is a mistake, the “In truth,,,” line should have followed “Now, the biggest misconception is that if a shooter has a .223 with a 1:8″ twist, his rifle won’t stabilize a 55gr bullet or anything lighter.”.