This post is all about finding the right twist rate for your rifle and ammunition.
I touched on the topic just a bit in my recommendations for a first time AR buyer and guide to AR-15 barrels, In both articles, I stated that the vast majority of AR-15 shooters should pick up something in a 1/7 or 1/8 twist rate and call it a day.
That advice still holds true. But I’ve also received emails from you guys asking about ideal twist rate for this caliber or that rifle. I imagine it’s probably a popular question.
I want to dig a little bit more into the science of barrel twist rates. Like my discussion of Mil-dots and MRAD, this will involve a bit of math up front, but I’ll simplify it for you in the end.
Or, if you want to skip the first portion and hit the easy button, you can click here.
Rifling and Bullet Stability
Let’s start with purpose of rifle twist rates all together.
The practice of carving grooves down the bore of a gun barrel dates back to at least the 15th century, but it was relegated mostly to cannons.
Applying rifle grooves to small arms grew to command a presence during the American Revolution.
At the time, most infantry small arms had smooth bores. This was a compromise allowing for faster loading from the muzzle. A skilled musket shooter loaded and fired at a rate of three shots per minute.
The .69 caliber 545-grain lead ball traveling at around 1000 feet per second would do significant damage to the human body.
If it hit, that is.
The problem with these smooth bore muskets was that they were inaccurate. The lead balls were prone to fly off in different directions after firing. But given the type of infantry tactics at the time, this wasn’t seen as an issue.
18th-Century infantry tactics relied on massed volleys of fire from formations of troops. That tactic put a premium on volume of fire, not accuracy.
Rifling for small did exist, which would have helped the situation, but it was impractical. Rifles require a tight fit between the lead ball and the bore of the rifle. Without that tight fit, the projectile won’t spin or the expanding gasses might blow by it entirely.
This tight fit also meant that the rifle was much slower to load and fire.
Enter the “OG” American Sniper
Gunsmiths started producing the first iterations of Colonial American rifled weapons in during the early 1700s. The Long Rifle, later known as the Kentucky Long Rifle, was a primarily frontier weapon used for accurate hunting and defense.
I’m not venturing down the history of the revolution. But I’ll simply say that those pesky colonists put their hunting rifles to great use during the war.
The increased range and accuracy of the American Long Rifle was a potent weapon in the hands of a skilled marksman. If you are interested in this portion of the war, read up on Daniel Morgan’s Riflemen.
It wasn’t until the mid 19th century that rifling became commonplace with infantry small arms, thanks to breech-loading mechanisms.
How Rifling Works
Rifling imparts rotational force to the projectile. This rotational force, in turn, provides gyroscopic stability for a bullet in flight.
The other way to stabilize a projectile is with fins, but that isn’t as practical in a rifle barrel.
To illustrate this, think of throwing a football. Without spin, the football tumbles end over end until it lands some relatively short distance away. However, if you can throw it with spin, the ball becomes much more accurate and stable in flight.
Bullets work the same way.
In ballistics, the center of pressure is the point where all of the aerodynamic forces, particularly lift and drag, are equalized. If this point is behind or below the center of gravity, then the object is self-stabilizing.
If the center of pressure is in front or above the center of gravity, as with a bullet, then the lift and drag forces induce torquing effect.
Spinning the bullet along its long axis prevents this tumbling.
You may have heard the team “keyholing” on a target. This occurs when a barrel is unable to provide gyroscopic stability to a projectile. The bullet tumbles end over end because aerodynamic forces are winning out. It continues tumbling until it hits the target.
The hole left behind appears elongated since the bullet passed through sideways.
Like throwing a football without spin, this results in extremely degraded accuracy and range for your rifle.
When you start seeing this with a rifle then the barrel is either “shot out” or the twist rate is totally mismatched to the ammunition.
Matching Rifle Twist Rate to Caliber
Now you know that rifling is important for its added stability. But where things start to go off the rails is the engineering behind how fast a given bullet needs to spin in order to remain stable.
We write the twist rate in terms of one rotation over X inches of barrel. So, when you see 1/7 twist, as found on mil-spec AR-15 barrels, that means one rotation for every seven inches of barrel.
On a 20″ barrel, that means the bullet rotates nearly three full turns before exiting. If that were a 20″ .308 barrel with the common 1/10 twist, the bullet would rotate twice.
For context, the Kentucky Long Rifle used by Morgan’s Riflemen had a twist rate between 1/60 and 1/70.
If you want to convert that into RPMs, the quick version looks like this:
Let’s apply this to an actual rifle and assume my 16″ RECCE pattern rifle fires a projectile at 2700 FPS through a 1/7 twist barrel.
As you can see, the projectile spins at 277,714 RPM as it exits the barrel. That’s interesting but not really helpful.
What we’re really after is figuring out the ideal twist rate for my rifle.
Gyroscopic Stability Factor
We measure how stable a bullet is in flight by assigning a gyroscopic stability factor. Any value below 1 is unstable. Between 1 and 1.3 is marginally stable, and anything higher than 1.3 is stable.
There are some caveats here, though.
These stability values are assumptions based on generic bullet designs. For example, Brian Litz did a lot of research using VLD bullets popular in long range competition. These bullets tend to change their stability factor as velocity decreases.
So, his conclusion was that you should target the 1.5+ range for your long-range shooting needs.
As stability factors decrease below 1.5, the bullet’s ballistic coefficient starts to decay. You should expect a 3% BC loss for every 0.1 stability factor loss below 1.5.
So how do we figure this out? There are three formulas I’ll show you. Each one builds upon the work before it.
The Greenhill Formula
Sir Alfred Greenhill, a British mathematician, developed this formula in 1879. It worked well enough in the 19th century for lead core bullets but doesn’t cut it for modern precision.
The formula did, however, provide a foundation for further development. This is a summarized version that’s a little easier to work with.
- C = 150, or use 180 for muzzle velocities higher than 2,800 f/s
- D = the bullet’s diameter measured in inches
- L = the bullet’s length in inches
- SG = specific gravity, a factor unique to each bullet
The Miller Formula
Don Miller developed this formula as a more accurate way to find an ideal rifle twist rate. Whereas Greenhill was more of a generic formula, Miller’s uses more characteristics of a rifle bullet to arrive a a more tailored solution.
- m = bullet mass in grains
- s = gyroscopic stability factor (dimensionless)
- d = bullet diameter in inches
- l and L= bullet length in calibers
If you’re scratching your head about that “length in calibers,” that’s ok.
Take the length of the bullet and divide it by its width. For example, a 175 grain SMK .308 bullet is 1.24″ long. So we divide 1.24 by .308 and get 4.026 calibers in length.
Let’s apply the formula for our 175 gr SMK bullet. I’m going to insert a gyroscopic stability factor of 1.5 for s, based on Litz’s recommendation.
If you punch all of that into your scientific calculator, you end up with a twist rate of 12.8 inches per turn. So, the short version is that you’d want a minimum 1/12 twist barrel.
You can also turn this around to find a stability factor if you already know the twist rate for t.
Let’s figure out the stability factor for a 175 gr SMK fired in a 1/10 twist barrel.
That works out to a stability factor of about 2.4, which is great for precision shooting.
For simplicity, this formula assumes a nominal muzzle velocity of 2800 fps and a temperature of 59 degrees. Those are generic numbers used by the Army for ballistics.
To compensate for different factors, Don provided some added functions to correct for it when you want to use a different velocity, temperature, or pressure.
Here’s what it looks like if you want to insert a new velocity (V).
Now we’re making progress. You might notice that the caliber is cubed in this one. It’s actually a slightly different variation where you supply twist rate (t) as calibers per rotation.
Don’t worry, I’m going to give you the easy button in a few minutes.
Improved Miller Formula
This version came about through a collaboration of Don Miller and Michael Courtney for Precision Shooter.
A lot of new bullet designs include polymer tips to aid with aerodynamics. With these projectiles, using the total length of the bullet makes calculations inaccurate. The mass does not distribute the same way as Miller’s earlier assumptions.
To compensate, the pair made an adjustment to the original work:
In the lower part of the formula, Lm signifies the length of the metal portion of the bullet, not including the polymer tip.
Putting the Math Together
Here’s how the whole thing looks when put together.
- S = stability factor
- m = mass in grains
- t = twist rate in calibers per inch (do twist rate divided by caliber)
- d = caliber
- L = the length of the bullet in calibers (bullet length in inches divided by caliber)
- L sub m = length of the metal portion of the bullet in calibers
- V = actual velocity
- FT = actual temperature in degrees Fahrenheit
- P = actual barometric pressure in inches of mercury
Shortcutting the Math
You’ll also want to use their library of bullet lengths. I assume they use the improved stability formula above since they include the plastic tip length in calculations.
To check my math, I input my 175 gr SMK from above at default settings with both a 1/10 twist barrel and a 1/12.8 twist barrel to see what popped out.
Right on the money.
Over-Spinning and Terminal Effect
I want to bust a couple myths here for good measure.
There is an optimum twist rate for a barrel given specific projectile. But a lot of people worry about over-spinning a bullet.
For example, it’s commonly known that a 1/7 twist rate is good for 62 gr M855 as well as 77 gr SMK, but a lot of people think it’s too fast for a 55 gr M193.
They also think it’s excessively fast for a lighter 45 gr varmint bullet.
So here’s the truth. Spinning a bullet too fast might degrade your accuracy just a little bit.
I emphasize might.
The faster twist rate induces slightly more spin drift and yaw.
Spin drift is the tendency of the bullet to travel horizontally in the direction it’s spinning. It really only shows up at very long distances. For practical purposes, you really can’t overspin a bullet from an accuracy perspective.
That said, small lightweight bullets with thin jackets might self-destruct in mid-air if spun too quickly. Varmint shooters usually point this out with their lightweight 45 gr bullets fired from fast twist barrels.
To keep this short, you should aim for a stability factor between 1.5 and 2. Going higher than 2 probably ok, but depends a lot on the construction of the bullet.
The only way to know if it works for you is to test it.
Tumbling and Terminal Effect
Some people assume that a marginally stable bullet in flight is more likely to tumble and fragment on impact.
Don’t fall for it, though.
The nature of a projectile like 5.56 to tumble and fragment is not related to its aerodynamic stability. Rather, it’s the result of the impact itself.
Remember when I mentioned the center of pressure vs the center of gravity above? When the bullet impacts, the dramatic increase in drag moves the center of pressure way in front of the center of gravity. At the same time, the deceleration and reduction in RPM from friction further destabilize the bullet.
At that point, the bullet can’t help but tumble. It has little to do with how stable it was flying beforehand.
A slight angle of attack on impact may speed up the tumbling effect, though. This angle of attack is a slight deviation of the tip of the bullet from the centerline of flight.
This happens naturally during spin stabilization, but the tip will be further away from the center at different times. You can’t control where the tip is pointing at any given time, however, so don’t worry about it.
If you want to read more about how bullets actually wound and kill, check out my article on terminal ballistics.
Twist Rate Cheat Sheets
Now to the easy button. Based on the formulas above, I put together some charts to help you in the future. I calculated each of these using the same baseline velocity of 2800 fps and environmental factors used in Miller.
In the case of 300 BLK, though, I applied corrections based on what I know about those velocities.
I’m including representatives from each of the main bullet classes here. I also brought in the 90 gr SMK popular with the .224 Valkyrie cartridge.
From the looks of the numbers, you can see why I advocate that most shooters are just fine with a 1/7 or 1/8 twist barrel for their AR-15 rifles.
I know the 1/7 with 55 gr shows 3.6, which is high, but it continues to function well for me. Since I usually shoot 62, 75, or 77 gr ammo though, a 1/7 works great as an all-rounder. Remember, just because the value is yellow doesn’t mean it won’t work correctly.
.308 Win and 7.62 NATO
In the .308 Win class, I thought the results were interesting. The bullet seems pretty stable across all of the popular .308 twist rates. The 1/11 looks to be the best all-around twist rate here with the 1/10 becoming more important if you are a hand-loading VLD bullets.
Shot at subsonic velocities, 300 BLK is very quiet and packs a punch. At supersonic levels, it’s a worthy contender to the classic 30-30 or 7.62×39 found in the AK.
The trouble is that users have to choose whether or not they’re going to focus mainly on subsonic or supersonic.
Granted that there is some leeway in the calculations, but it appears to me that there isn’t an ideal “do all” barrel twist rate for 300 BLK. If you want it to do very well at supersonic loads, then it’s going to do poorly with subsonic loads.
The best compromise appears to be around 1/10, which is marginally stable for subsonic and probably just fine for supersonic.
Of course, that’s if you’re trying to avoid the yellow zone. A 1/8 twist rate would probably be just fine for shooting both super and subs. Your mileage may vary, of course, depending on your experiences.
Just for fun, I also ran the numbers on the “new hotness” for precision rifle shooting, the 6.5 CM.
Like the .308, most twist rates seem to do a pretty decent job. If I was shopping around, the 1/8 twist seems to be the ticket.
Alright, I’m done nerding out for the day.
At this point, you’ve got a sense of why rifling exists. Since the aerodynamics of bullet flight want to make the projectile tumble end over end, we need a way to stabilize it. Fins aren’t practical for small arms, so we use spin stabilization.
Spin is imparted by the rifling grooves down your barrel. The rate at which these grooves curve around the bore, the twist rate, imparts many thousands of RPM to the bullet. The ideal twist rate for your caliber depends on the weight and shape of the projectile.
There are a lot of formulas out there for figuring out the right twist rate, and I walked through the important ones with you. I also provided a few links to online calculators that do a great job. Lastly, I left you with a few charts to work with in the future.
If you have any more questions or want any more charts for specific calibers, let me know in the comments.