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 in popularity 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 smoothbore 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 the volume of fire, not accuracy.
Rifling for small arms 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. This is why fin stabilization works, since it pulls the center of pressure much further rearward.
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 where the projectile wants to tumble end over end. 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, 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 recommended minimum.
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 since 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
The fastest way to get your numbers is to use JBM Ballistics. I’ve referenced them before while discussing point blank zeroes and trajectories. They also have a handy stability calculator.
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.
Over-Spinning
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.
.224 Caliber
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
I thought the results were interesting for the 30 cal. 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.
300 Blackout
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.
6.5 Creedmoor
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.
Wrapping Up
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.
I think a chart of 6.5mm/.264 cal barrel twists wold be interesting, but I don’t know of a company selling a 6.5 CM that isn’t aimed at long range using heavy bullets, so I don’t know how useful it would be.
Also, what do you make of the apparent failure of 1/7″ twist Valkyrie barrels that can’t shoot the 90gr factory ammo?
I’ll try to work up a 6.5 chart this weekend. The thing to keep in mind is that there’s more to 6.5 than the CM. For example, 6.5 Swede (6.5×55) has practically been around longer than 30-06. It’s a great hunting round, and a lot of hand loaders like it because they can cram the powder in the case. Only drawback was that it needed a long action.
I hadn’t heard of the 1/7 twist failures with the Valkyrie. After some quick googling, it seems to be hit or miss. A lot of companies are working towards a 1/6.5 or 1/6.7 twist. Interestingly, a few mentioned that 1/7.5 was working fine with longer barrels.
My suspicion is that the 1/7 failures are coming from relatively short barrels and reduced velocities. It might also be some other geometric factor with 90gr SMK not easily worked into the Miller formula. Unfortunately, that’s why it’s a tool in the toolbox but will always be trumped by real world experience.
I recently bought a 1920 Carl Gustafs M96 Long Rifle ie 29in bbl and have picked up a couple different 140 gr bullets from hornady, remington and Speer, as well as some hornady 160gr RN. Will be reloading with some IMR4831 and RL22. I should be able to get the 140s up to about 2700-2800fps and the 160s to around 2300-2500fps. The M96 has a twist rate of 220mm ( 7.87 in. ) and looks to be quite sufficient. My worry is I also obtained some 170gr GC cast bullets, and my research says I shouldn’t go beyond about 140,000rpm which translates to 1500fps, will I still be able to stabilize these?
Hi! Thanks for putting out this content, I enjoy reading it a lot. Do you have a chart for the 6,5 Swede (6,5×55)? I am about to purchase a rifle in this caliber soon, and I would like to check the chart before buying ammunition. Will be used for competitions (30 rounds on various animal paper targets, ranging from 50 to 220 meters).. According to my Tikka manual and Tikka.fi website, the twist rate for Tikka rifles chambered in 6,5×55 is 1:8. I have a Tikka T3 Supervarmint SS in .308 Win, with a twist rate of 1:11, which according to your chart is good
Hi Kjetil, my understanding is that the 6.5×55 and 6.5 Creedmoor shoot essentially the same bullets. Since twist rate is based on the projectile itself, I think the chart I have here for 6.5 Creedmoor applies and the 1:8 twist is probably the most useful.
I agree. After doing some research, which I should have done earlier, I see that the bullets used are more or less the same. I thought they would differ due to the different case length and seating depth. The 6,5×55 case length is appr. 6,2mm longer, and the case capasity is slightly larger on the Swede. Thanks for the quick response, highly appreciated
I found these online; Hornady Frontier Ammo 223 Rem 68Gr Bthp. https://www.jaktogfriluft.no/jakt/ammunisjon/rifle-ammo/hornady-frontier-ammo-223-rem-68gr-bthp
Do you agree they will be running fine in an Armalite M15 18 inch .223 Wylde 1:8 twist?
Hi, Matt
Can I please ask you to extend the .308 Win chart in both directions for bullet mass? I am shooting Federal 110 gr V-Max bullet, Vor-Tx Barnes 130 gr TTSX BT, and the Lapua 200gr subsonic FMJBT. I would like to see whether these are within spec.
So I worked up some quick back-of-the-napkin numbers for 6.5/.264. I ran it from 120gr up through 155 gr (as you might find in the long action 6.5 Swede cartridge). I ran it with 2800 fps and all the standard temperature/pressure estimates I did above.
From the look of it, I think somewhere between 1/7 and 1/8 is a nice balance. I thought it was interesting that the Nosler behaved differently. JBM’s length numbers might be off because the Nosler 130gr is the same length as the Sierra 140gr SMK.
Intelligent information provided with clarity – illustrates understanding and the ability to communicate effectively. Thank you for this.
FYI – I shoot a Savage 12 FVL – Heavy Barrel .223 with a Vortex Viper 6x20x44 scope via a bi-pod and butt sandbag. In general, my set-up prefers the 60-77 grain range of bullet – most likely to go with the 69 grain Sierra. Average 5-shot groups over the past year have an average MOA of 0.80 with the best at 0.30 MOA – but this likely had some luck Just moving into re-loading for the 223 with Nosler brass and CCI Benchrest primers.
Hi Dwight, thanks for stopping by! That sounds like a fine shooter you’ve got there!
Thanks – pleased with the Savage. Up here in Canada we are all trying to emulate the Canadian JTF2 snipers in Afghanistan and Iraq – albeit shooting at maybe 1/10 their distances.
I shoot many calibers and 54 cal round ball out of a twist rate of 1in 72 and at 200 yard with a 12 inch gong and can make it ring often
Something interesting to think on: the Army’s new M110A1 CSASS 308 DMR uses a 1:8 twist on a 16.5” barrel…and here I was sweating that my 1:10 16” Criterion was going to be too much twist to stabilize the lighter bullets. The British L129A1 uses a 1:11.25 twist on a 16” barrel because they primarily shoot 155gr match ammo but still require the use of 147gr-ish ball (including delinked MG ammo). Keep in mind also that the M110A1 is not a sniper rifle and will mostly be fed light M80A1 ammo instead of 175gr M118LR.
Excellent example! Again, I think most people overthink the whole overtwist issue. Is there some situation where running a too-fast twist going to be a detriment? Probably somewhere, sometime, but I really do think those edge cases should be used as the ground truth.
Hi! First off, thank you for all the great articles that you have written.
Where I am from, a lot of old timers said that it is dangerous to shoot the M855 out of the old M16 barrels. They said that doing so would result in a catastrophic blow-out. My question is, where is this idea coming from?
Hi John, thanks for coming by and asking!
To answer your question, I’m not sure where that idea comes from, but it is incorrect. When talking about old M16 barrels, I assume they’re referring to 1/12 twist M16A1 barrels. The old 3-22.9 field manual stated that shooting M855 through those barrels should only be done in an emergency and at distances of less than 90 meters. The reason is that the 1/12 twist barrel does not stabilize the heavier SS109 bullet (you can see in my chart above that it results in a .8 stability factor, which is “unstable”).
That combination, while inaccurate, is not dangerous from a pressure standpoint. In fact, the slower 1/12 twist may actually reduce the pressures in the chamber.
Now, all of that said, I suppose there might be confusion stemming back to chamber dimensions. The 5.56 NATO cartridge and chamber didn’t come around until much later. The old chambers used for M193 were slightly different, as updates had to be made to help with the feeding of new tracer rounds. I suspect that this is less about M855 vs M193 than it is about NATO pressure rounds fired from .223 rem chambers.
The standard wisdom is that it’s a bad idea. The probability is that everything will be just fine, but most people like to play it safe.
Not sure how the tables for 224 stability follow from the Miller formula: that formula shows that the stability factor **increases linearly ** with mass m of the bullet, but the tables show it decreasing non-linearly.
Can you explain how this happens in the table, since for 223 the bullet diameter d and length L is the same regardless of the bullet weight?
Hi jacks, I’m not sure I understand the question. But the length of the bullet does change as the mass increases. Since it cannot increase in diameter (it always has to be .224), then the bullet must increase in length to accommodate the extra mass.
For the calculations, I used a library of bullets from JBM Ballistics, which include the lengths
If you look at Miller’s formula in your write up, stability factor = m * (bunch of stuff that depends only on twist, diameter, Length but NOT on mass m).
So as m increases for the same bullet dimensions and twist rate, the stability factor goes up. But in your table for 1:9 twist, it goes down.
Bullet mass can change without affecting the bullet dimensions, for example by varying the mix of metals that make it up (for example, more copper, less lead).
Are you reading the table vertically or horizontally? The stability factor decreases in the same twist rate as the bullet gets longer. That’s nearly true across the board. If you have an alternative to test, I’m happy to put it up.
As far as bullet composition, you’ll have to provide some examples. I only listed the most commonly discussed bullets, and there are a lot out there. That said, even the all-copper Barnes bullets tend to need faster twist rates as they are less dense for weight and so are usually longer.
I’m reading the 224 table vertically: for the 1:9 twist I see
weight stability
45 3.0
55 2.2
62 1.5
I know the stability factor decreases as the bullet gets heavier but that is **not** what is predicted by the Miller formula, as mentioned above. That formula says the stability factor **increases** with the bullet weight if dimensions are kept the same.
[ Using L = 3.4 for 223, I get 2.2 for 55 grains using the Miller formula but 2.4 for 62 grains.
Would need L = 4 to get 1.5 for 62 grains, meaning a 62 grain bullet would be 0.89″ long vs .75″ for a 55 grain one]
The dimensions of the bullet do not stay the same as the bullet gets heavier, though. I would have to go back and revisit the JBM bullet library for specifics, but your example is correct.
A 55gr M193 bullet is .76” long, while the 62gr M855 is .906” long. The Sierra 77gr SMK is .955” and the all-copper Barnes 70gr TSX is 1.037”
If the bullet length changes with the weight, instead of by changing the mix of metals, then it would be more accurate to say that the length L depends on the mass m.
Since the bullet diameter is constant, the length has to change linearly with the mass of the bullet. A linear fit for the case of 223 55 and 62 grain bullets would be:
L = (1/d) * (.75 + 0.025 (m – 55) where d = caliber = 0.223
Then when you look at Miller’s formula, you can see that the stability changes as 1/sqrt(m) roughly, so that’s why it goes down as weight goes up.
“Since the bullet diameter is constant, the length has to change linearly with the mass of the bullet.
This in incorrect, a plain base bullet will be shorter than a high BC boat tail bullet of the same weight. The ogive can also be increased to make a longer pointier nose to also increase the length.
You are correct, there are variations to geometry at play here- but as a general rule, this helps most people understand the relationship between bullet weight and twist rate.
When I plug numbers into the equations, I think I am missing something. Calculating stability factor, you used straight twist, i.e., 1/10″ twist rate, t=10. But, when you put it all together with velocity, temp & pressure, t becomes twist rate in calibers. Plus, the d is raised to the third power when it was not cubed in the basic equation. Where did those two changes come from?
Hey Dean, thanks for commenting. Now that you mentioned it, I think I made a typo in some of the graphics (I originally wrote this article in 2018, and the calculator I built to run the numbers appears long gone). From looking over my sources, the “d” should have been cubed the whole time. I must have left it out of the basic Miller Stability formula by accident.
My question is more to do with barrel length, do bullets seem to stabilize better in longer barrels or does it not matter? My thought is that it’s all about velocity with a shorter barrel but my friends seem to think a longer barrel will stabilize the bullet better and be more accurate because of it.
Hey Travis, thanks for asking. Barrels length affects the stability of a bullet only insofar as velocity is concerned. Velocity has an effect on stability, but it’s very minor compared to the twist rate.
Bryan Litz, in Applied Ballistics for Long Range Shooting, pointed out that increased velocity may add to the RPM for better stability, but that also increases force applied to the nose of the bullet as it spins and increases aerodynamic torque (meaning less stability). The two opposing forces basically cancel each other out.
Does barrel length play into this at all? I am customizing my T3x CTR in .308. Example: would you want a 1:9 twist on a 19” barrel when you would have a 1:11 in a more standard 22-24” length?
Hi Michael, thanks for asking. Barrel length only plays a factor insofar as velocity is concerned. If you use this formula here:
The length of the barrel only affects the “V” (for velocity) in the middle portion. You can play with different values here to reflect longer or shorter barrels, but you’ll find that the length of the barrel has a very minor effect on the stability factor. So, in short, it’s not something to worry about here.
On the other hand, the barrel length (velocity), has a much more impact on how the bullet reacts to wind and gravity.
Thank you. I have heard that .308 has the same accuracy and distance with a 20” barrel as it would 24” due to harmonics etc. Would you say this is accurate? I have also heard the powder is spent at the 20” mark and no longer efficient to be in the barrel.
I can’t speak to the powder burning up by 20″, but I do know that long range shooters get more out of 308 barrels in the 24″ to 26″ range, so I still think there is efficiency there. While true that the accuracy and maximum range between a 20″ and 24″ is not really changed, what you gain is ballistic efficiency against wind deflection at long ranges.
For LE/mil sniper types, they report no real downside to shorter barrels in 308 because they are primarily focused on mid range (200-800 meters), and would rather have the benefits of compactness and lighter weight over improved ballistic efficiency at 1000k+
In the end, it comes down to what you want to use it for.
I believe the powder burning up by. 20 inches depends on the powder being used. A slow burning powder will need a longer barrel to burn up. A fast powder only needs a short barrel.
Thanks for the 300blk info, just building a custom rifle and only shoot 110g up to 150g so trying to find the right twist rate, not much info out there on the blk. Thanks heaps might go for a 1/12 twist I think
You’re very welcome, Donovan. Enjoy the build!
.35 Whelen ?
I have 308 with a 22″barrel an a 1/12 twist try to find the right bullet weight to accurately shoot?
Hi Corey, with a 1/12 you should be fine with anything up to 175gr
Thank you for the great information, I have been shooting a long time, but this post brings forth a lot of missed information.
NOW I have a good excuse for missing.
I’m glad you enjoyed it, Fred!
Hi Matt. Have you tried using duplex or triplex loads? I have an 8×57 mauser that has a thoroughly pitted barrel, end to end! Wouldn’t shoot at all accurately with any factory or listed loads. I then figured up a starting load with three quite different powders. !!! First try and it was right on! Perfect pressure signs on primer and case. Accuracy was amazing! All five shot groups 100 to 300 yards were covered by a quarter! That year I missed the White tail because I took the time to count his antlers! 9 each side! Then he turned the opposite way at the end of the ridge and I shot the wrong way! Dam! But I got a mule deer the next year easily. The first shot tickled his belly fur. Next shot aimed1-1/2″ above his back shredded his heart. Good eating, but I would have preferred the White tail. Especially with that rack! Oh well. Maybe some other time.Well good shooting and experimenting!
I am looking at 280 and 280 AI sporting bolt action rifles. Many come with 1:9 twists. I have seen one with 1:7 twist. 22-24″ barrel lengths? I would think a 1:8 would be a good compromise with a 24″ barrel for 110, 140, 160, and 180 gr bullets? Any thoughts on the matter?
Hi Chris, I think your hunch is probably correct. A 1/8 sounds like an ok compromise to shoot a variety of bullet weights. That said, I’ve rarely seen a downside to using a twist rate on the higher end of the range.
Matt,
I know this is an old article, but I hope still get this question. I want to build a AR platform rifle for a specific style of match. Typically folks shoot this match with 22LR. I know there are a few ways to go about this, such as a 22LR upper or a conversion kit for my standard AR (1:7). I believe 22LR is more suited to slower twist rates (1:16), and using a conversion kit may not give the best results using cast/plated cast 22LR bullets. I don’t necessarily want a dedicated 22LR-only upper. I will mostly shoot it with 22LR, but I like the “option” of using 5.56. My question: What would be the effect of shooting 5.56 put of a 1:16 rifling? I ask because your charts all say if you dip into the red “you’ve got problems”. I have a source for a 1:16 barrel with a 5.55 chamber.
Hi Shannon, a 1:16 barrel will work fine for 22LR, but would probably struggle with anything but the lightest .223 bullets, and probably wouldn’t be very useful for most things people would use an AR for.
The alternative is to try and find something like a 1:14 barrel and see how well 22LR shoots out of it.
Matt, what is the heaviest bullet you would advise sending down a 12.5″ 1:10 twist 308 barrel? Thank you!
A 1/10 should honestly work with just about anything that fits in at mag length. My chart went up to 175 & 180 gr, and both would work fine. The length of the barrel being 12.5″ doesn’t actually affect the stability very much, only the velocity.
Ahoj kupil som Brenner BR20 win308 dlžka 51cm twist 1:11 ake strelivo/guľky je vhodne pre ten kaliber do vzdialenosti 100m Ďakujem
Hi Matt
I can not get clear answers on the internet or any forum, and your formulas seems very complicated…can you please help me with the following regarding the right barrel twists for some calibers:
1) Can a 375 Ruger with a 23 inch barrel with a 1:12 twist, shoot 350 grain bullets? (Will it stabilize)
2) Can a 35 Whelen with a 24 inch barrel with a 1:14 twist stabilize 200 – 310 grain bullets?
3) Can a 358 Norma Magnum with a 24 inch barrel with a 1:14 twist stabilize 200 – 310 grain bullets?
Hope to hear from you
Thank you
Hi Michael, to get a really good answer you have to provide more data about the bullet itself- particularly the length. Just saying the cartridge and grain isn’t enough. If you have that, you can plug it into the calculator I linked to in the article at JBM Ballistics.
That said, I went ahead and took a best guess based on looking for some bullets that fit your description (i.e. 375 Ruger 350 grain). Without any other data like the specific bullet or velocity, consider these to be best guesses.
1) Yes, a 375 Ruger with a 350 grain bullet should stabilize in a 1/12.
2) 200 to 310 grains is a huge difference. By my numbers, a 1/14 is probably fine for both.
3) For a 358 Norma Magnum, I’m assuming that it’s basically the same bullets as the 35 Whelen since both are .358 diameter. In that case, the same rules apply and a 1/14 should be fine.
Hi Matt – I was just running your formulas through a spreadsheet and was not getting the correct result for the last formula “Putting the Math Together”. You have caliber cubed in the denominator, and in order to get the stability factor that you’re citing for the 1:10 twist rate, it just needs to be caliber.
Perhaps you can check, and modify the equation, in case anyone else is having such difficulty.
You forgot the most important 270..1in7 1in8 1in9 1in10
And 130grain 140 145 150 160 please and thank you