Many barrel-makers mark the twist rate and bore dimensions on their barrel blanks.
Does muzzle velocity change with faster or slower barrel twist rates? Absolutely, but much less than you might think. Faster twist rates do slow down bullets somewhat, but the speed loss is NOT that significant. With Bartlein .308 Win barrels of identical length and contour, a 1:12″-twist barrel was only 8 fps faster than a 1:8″-twist barrel. That was the result of testing by Applied Ballistics.
The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting, Vol. 1. That book includes other fascinating field tests, including a comprehensive chronograph comparison.
Barrel Twist Rate vs. Velocity — What Tests Reveal by Bryan Litz
When considering barrel twist rates, it’s a common belief that faster twist rates will reduce muzzle velocity. The thinking is that the faster twist rate will resist forward motion of the bullet and slow it down. There are anecdotal accounts of this, such as when someone replaces a barrel of one brand/twist with a different brand and twist and observes a different muzzle velocity. But how do you know the twist rate is what affected muzzle velocity and not the barrel finish, or bore/groove dimensions? Did you use the same chronograph to measure velocity from both barrels? Do you really trust your chronograph?
Summary of Test Results
After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the average rate of approximately 1.33 FPS per inch of twist. In other words, your velocity is reduced by about 5 FPS if you go from a 1:12″ twist to a 1:8″ twist. — Bryan Litz
Savage Test Rifle with Six Bartlein Barrels
Most shooters don’t have access to the equipment required to fully explore questions like this. These are exactly the kinds of things we examine in the book Modern Advancements in Long Range Shooting, Vol. 1. In that book, we present experiments conducted in the Applied Ballistics lab. Some of those experiments took on a “Myth Buster” tone as we sought to confirm (or deny) popular pre-conceptions. For example, here’s how we approached the question of barrel twist and muzzle velocity.
Six .308 Win Barrels from Bartlein — All Shot from the Same Rifle
We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations. In this way, we’re truly able to compare what effect the actual twist rate has on muzzle velocity with a reasonable degree of confidence.
Prior to live fire testing, we explored the theoretical basis of the project, doing the physics. In this case, an energy balance is presented which predicts how much velocity you should expect to lose for a bullet that’s got a little more rotational energy from the faster twist. In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist (e.g. a 1:8″ twist is predicted to be 1.25 fps slower than a 1:9″ twist).
Above, data shows relationship between Twist Rate and Muzzle Velocity (MV) for various barrel twist rates and rifling types. From fast to slow, the three 1:10″ twist barrels are: 5R (canted land), 5 Groove, 5 Groove left-hand twist.
We proceeded with testing all 6 barrels, with twist rates from 1:8″ to 1:12″. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the average rate of approximately 1.33 fps per inch of twist. In other words, your velocity is reduced by about 5 fps if you go from a 1:12″ twist to a 1:8″ twist. [Editor: That’s an average for all the lengths tested. The actual variance between 1:12″ and 1:8″ here was 8 FPS.] In this case the math prediction was pretty close, and we have to remember that there’s always uncertainty in the live fire results. Uncertainty is always considered in terms of what conclusions the results can actually support with confidence.
This is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting Vol. 1 is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.
Other chapters in the book’s twist rate section include: · Stability and Drag — Supersonic
· Stability and Drag — Transonic
· Spin Rate Decay
· Effect of Twist rate on Precision
Other sections of the book include: Modern Rifles, Scopes, and Bullets as well as Advancements in Predictive Modeling. This book is sold through the Applied Ballistics online store. Modern Advancements in Long Range Shooting is also available as an eBook in Amazon Kindle format.
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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.
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Creedmoor Sports has recently expanded and updated the Creedmoor Sports InfoZone. This web resource contains valuable information about reloading, rifle maintenance, marksmanship skills, and gunsmithing. In addition, the InfoZone has Competition Reports, numerous Gear Reviews, and general Shooting Sports News.
For this Saturday Showcase, you’ll find five InfoZone Videos hosted by Bill Gravatt, President of Creedmoor Sports. There are over 85 other videos on the Creedmoor Sports YouTube Channel. Bill Gravatt is an expert on reloading processes and gear. He developed many of the popular tools for Sinclair Int’l, and now he directs operations for Creedmoor Sports.
Reloading Procedures with Bill Gravatt
Here are five helpful videos Bill Gravatt created for the Creedmoor Sports InfoZone. These videos cover a variety of essential processes for handloading. In addition Bill Shows how to find the exact twist rate in your rifle barrels.
How to Determine a Barrel Twist Rate
Bill Gravatt of Creedmoor Sports demonstrates an easy method to determine your barrel’s twist rate. Knowing the twist rate is essential to selecting suitable bullets for your discipline.
How to Seat Bullets with Inline Press
Many top Benchrest and F-Class shooters favor inline presses for bullet seating. With good hand dies (such as Wilson) and a inline Arbor press (such as a 21st Century), you can get great results with super seating depth consistency and very low run-out.
How to Trim Cases with L.E. Wilson Trimmer
There are many ways to trim cartridge brass. If you need to trim hundreds of cases it may pay to have a dedicated power trimmer. However, for most rifle shooters, a good hand trimmer will do the job very well. We regularly use a Wilson trimmer to trim our cases, as demonstrated in this video.
How to Clean Cartridge Cases
It is important to regularly clean cartridge brass. There are many different means. Wiping the outside with a solvent is a good start. For more thorough cleaning inside and out tumbling or wet-tumbling will do the job. After tumbling in media, you should always check the primer pockets and flash holes before loading.
How to Clean Reloading Dies
Cleaning your reloading dies is something that many who reload often neglect. In this 60-second Tech Tip, Bill Gravatt provides some tips on cleaning your reloading dies.
BONUS Video: How to Fine Tune Seating Depth
Now that you’ve learned Reloading Basics from the Creedmoor Sports videos, here’s a great video from our friend Keith Glasscock that will help you get optimal results when seating bullets. Keith is one of America’s greatest F-Class competitors, with multiple podium finishes at F-Class National Championships. With over 213,000 views, this is the most popular video Keith has created for his Winning in the Wind YouTube Channel.
About Creedmoor Sports — 45 Years of Service to Marksmen
For the past 45 years, since 1979, Creedmoor Sports has been supplying quality equipment and supplies to avid shooters in a wide variety of disciplines: Rimfire, F-Class, PRS/NRL, Silhouette, Service Rifle, Benchrest, and more. Specializing in competitive centerfire, smallbore, and air rifle accessories, as well as reloading components and tools, Creedmoor Sports seeks to provide its customers with the right gear to help them achieve their personal shooting goals.
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Many barrel-makers mark the twist rate and bore dimensions on their barrel blanks.
Does muzzle velocity change with faster or slower barrel twist rates? Absolutely, but much less than you might think. Faster twist rates do slow down bullets somewhat, but the speed loss is NOT that significant. With Bartlein .308 Win barrels of identical length and contour, a 1:12″-twist barrel was only 8 fps faster than a 1:8″-twist barrel. That was the result of testing by Applied Ballistics.
The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting, Vol. 1. That book includes other fascinating field tests, including a comprehensive chronograph comparison.
Barrel Twist Rate vs. Velocity — What Tests Reveal by Bryan Litz
When considering barrel twist rates, it’s a common belief that faster twist rates will reduce muzzle velocity. The thinking is that the faster twist rate will resist forward motion of the bullet and slow it down. There are anecdotal accounts of this, such as when someone replaces a barrel of one brand/twist with a different brand and twist and observes a different muzzle velocity. But how do you know the twist rate is what affected muzzle velocity and not the barrel finish, or bore/groove dimensions? Did you use the same chronograph to measure velocity from both barrels? Do you really trust your chronograph?
Summary of Test Results
After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the average rate of approximately 1.33 FPS per inch of twist. In other words, your velocity is reduced by about 5 FPS if you go from a 1:12″ twist to a 1:8″ twist. — Bryan Litz
Savage Test Rifle with Six Bartlein Barrels
Most shooters don’t have access to the equipment required to fully explore questions like this. These are exactly the kinds of things we examine in the book Modern Advancements in Long Range Shooting, Vol. 1. In that book, we present experiments conducted in the Applied Ballistics lab. Some of those experiments took on a “Myth Buster” tone as we sought to confirm (or deny) popular pre-conceptions. For example, here’s how we approached the question of barrel twist and muzzle velocity.
Six .308 Win Barrels from Bartlein — All Shot from the Same Rifle
We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations. In this way, we’re truly able to compare what effect the actual twist rate has on muzzle velocity with a reasonable degree of confidence.
Prior to live fire testing, we explored the theoretical basis of the project, doing the physics. In this case, an energy balance is presented which predicts how much velocity you should expect to lose for a bullet that’s got a little more rotational energy from the faster twist. In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist (e.g. a 1:8″ twist is predicted to be 1.25 fps slower than a 1:9″ twist).
Above, data shows relationship between Twist Rate and Muzzle Velocity (MV) for various barrel twist rates and rifling types. From fast to slow, the three 1:10″ twist barrels are: 5R (canted land), 5 Groove, 5 Groove left-hand twist.
We proceeded with testing all 6 barrels, with twist rates from 1:8″ to 1:12″. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the average rate of approximately 1.33 fps per inch of twist. In other words, your velocity is reduced by about 5 fps if you go from a 1:12″ twist to a 1:8″ twist. [Editor: That’s an average for all the lengths tested. The actual variance between 1:12″ and 1:8″ here was 8 FPS.] In this case the math prediction was pretty close, and we have to remember that there’s always uncertainty in the live fire results. Uncertainty is always considered in terms of what conclusions the results can actually support with confidence.
This is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting Vol. 1 is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.
Other chapters in the book’s twist rate section include: · Stability and Drag — Supersonic
· Stability and Drag — Transonic
· Spin Rate Decay
· Effect of Twist rate on Precision
Other sections of the book include: Modern Rifles, Scopes, and Bullets as well as Advancements in Predictive Modeling. This book is sold through the Applied Ballistics online store. Modern Advancements in Long Range Shooting is also available as an eBook in Amazon Kindle format.
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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.
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Many barrel-makers mark the twist rate and bore dimensions on their barrel blanks.
Does muzzle velocity change with faster or slower barrel twist rates? Absolutely, but much less than you might think. Faster twist rates do slow down bullets somewhat, but the speed loss is NOT that significant. With Bartlein .308 Win barrels of identical length and contour, a 1:12″-twist barrel was only 8 fps faster than a 1:8″-twist barrel. That was the result of testing by Applied Ballistics.
The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting, Vol. 1. That book includes other fascinating field tests, including a comprehensive chronograph comparison.
Barrel Twist Rate vs. Velocity — What Tests Reveal by Bryan Litz
When considering barrel twist rates, it’s a common belief that faster twist rates will reduce muzzle velocity. The thinking is that the faster twist rate will resist forward motion of the bullet and slow it down. There are anecdotal accounts of this, such as when someone replaces a barrel of one brand/twist with a different brand and twist and observes a different muzzle velocity. But how do you know the twist rate is what affected muzzle velocity and not the barrel finish, or bore/groove dimensions? Did you use the same chronograph to measure velocity from both barrels? Do you really trust your chronograph?
Summary of Test Results
After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the average rate of approximately 1.33 FPS per inch of twist. In other words, your velocity is reduced by about 5 FPS if you go from a 1:12″ twist to a 1:8″ twist. — Bryan Litz
Savage Test Rifle with Six Bartlein Barrels
Most shooters don’t have access to the equipment required to fully explore questions like this. These are exactly the kinds of things we examine in the book Modern Advancements in Long Range Shooting, Vol. 1. In that book, we present experiments conducted in the Applied Ballistics lab. Some of those experiments took on a “Myth Buster” tone as we sought to confirm (or deny) popular pre-conceptions. For example, here’s how we approached the question of barrel twist and muzzle velocity.
Six .308 Win Barrels from Bartlein — All Shot from the Same Rifle
We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations. In this way, we’re truly able to compare what effect the actual twist rate has on muzzle velocity with a reasonable degree of confidence.
Prior to live fire testing, we explored the theoretical basis of the project, doing the physics. In this case, an energy balance is presented which predicts how much velocity you should expect to lose for a bullet that’s got a little more rotational energy from the faster twist. In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist (e.g. a 1:8″ twist is predicted to be 1.25 fps slower than a 1:9″ twist).
Above, data shows relationship between Twist Rate and Muzzle Velocity (MV) for various barrel twist rates and rifling types. From fast to slow, the three 1:10″ twist barrels are: 5R (canted land), 5 Groove, 5 Groove left-hand twist.
We proceeded with testing all 6 barrels, with twist rates from 1:8″ to 1:12″. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the average rate of approximately 1.33 fps per inch of twist. In other words, your velocity is reduced by about 5 fps if you go from a 1:12″ twist to a 1:8″ twist. [Editor: That’s an average for all the lengths tested. The actual variance between 1:12″ and 1:8″ here was 8 FPS.] In this case the math prediction was pretty close, and we have to remember that there’s always uncertainty in the live fire results. Uncertainty is always considered in terms of what conclusions the results can actually support with confidence.
This is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting Vol. 1 is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.
Other chapters in the book’s twist rate section include: · Stability and Drag — Supersonic
· Stability and Drag — Transonic
· Spin Rate Decay
· Effect of Twist rate on Precision
Other sections of the book include: Modern Rifles, Scopes, and Bullets as well as Advancements in Predictive Modeling. This book is sold through the Applied Ballistics online store. Modern Advancements in Long Range Shooting is also available as an eBook in Amazon Kindle format.
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The U.S. Army Marksmanship Unit (USAMU), in cooperation with Shooting USA TV, has created a series of instructional Pro Tip pages covering a wide range of shooting disciplines. All totaled, there are more than 50 USAMU Pro Tips. Most relate to rifle marksmanship but there are also numerous tips for shotgunners and pistol shooters. Each Pro Tip entry includes multiple photos and 6-15 paragraphs, in an easy-to-follow format. Many Pro Tips also include an instructional video produced by Shooting USA. Here are three Pro Tip videos, and links to seven more Pro Tip web pages.
USAMU TOP TEN PRO TIPS
1. Reading the Wind with SGT Sherri Gallagher.
Apart from gravity, wind has the most pull on the bullet as it travels down range. Being able to accurately read the wind and mirage will greatly enhance your performance on the rifle range. National Champion, SGT Gallagher gives you some of her tips.
2. Angle Shooting with SFC (Ret.) Emil Praslick.
SFC Praslick shows you how to determine the angle to your target, and then how to include that to change your data necessary to hit your target on the first shot.
3. Rifle Grip, Stance and Body Position for 3-Gun with SFC Daniel Horner.
Professional 3-gun marksman SFC Daniel Horner, U.S. Army Marksmanship Unit (USAMU), give tips on how to properly handle a semi-automatic rifle, including grip, stance and body position.
Here’s an extreme range of .224-Caliber bullets: 35gr varmint bullet and 90gr match bullet. Of course, along with bullet length/design, you need to consider MV when choosing twist rate.
Even with the same caliber (and same bullet weight), different bullet types may require different rates of spin to stabilize properly. The bullet’s initial spin rate (RPM) is a function of the bullet’s muzzle velocity and the spin imparted by the rifling in the barrel. You want to ensure your bullet is stable throughout flight. It is better to have too much spin than too little, according to many ballistics experts, including Bryan Litz of Applied Ballistics. The late Glen Zediker provided some basic tips concerning barrel twist rates and bullet stability. These come from his popular book, Top Grade Ammo.
Choosing the Right Twist Rate
I’d always rather have a twist too fast than not fast enough. Generally… I recommend erring toward the faster side of a barrel twist decision. 1:8″ twist is becoming a “new standard” for .224 caliber, replacing 1:9″ in the process. The reason is that new bullets tend to be bigger rather than smaller. Don’t let a too-slow twist limit your capacity to [achieve] better long-range performance.
Base your next barrel twist rate decision on the longest, heaviest bullets you choose to use, and at the same time realize that the rate you choose will in turn limit your bullet choices. If the longest, heaviest bullet you’ll shoot (ever) is a 55-grain .224, then there’s honestly no reason not to use a 1:12″. Likewise true for .308-caliber: unless you’re going over 200-grain bullet weight, a 1:10″ will perform perfectly well.
Bullet Length is More Critical than Weight
Bullet length, not weight, [primarily] determines how much rotation is necessary for stability. Twist rate suggestions, though, are most usually given with respect to bullet weight, but that’s more of a generality for convenience’s sake, I think. The reason is that with the introduction of higher-ballistic-coefficient bullet designs, which are longer than conventional forms, it is easily possible to have two same-weight bullets that won’t both stabilize from the same twist rate.
Evidence of Instability
The tell-tale for an unstable (wobbling or tumbling) bullet is an oblong hole in the target paper, a “keyhole,” and that means the bullet contacted the target at some attitude other than nose-first.
Increasing Barrel Length Can Deliver More Velocity, But That May Still Not Provide Enough Stability if the Twist Rate Is Too Slow
Bullet speed AND barrel length have an influence on bullet stability, and a higher muzzle velocity through a longer tube will bring on more effect from the twist, but it’s a little too edgy if a particular bullet stabilizes only when running maximum velocity.
My failed 90-grain .224 experiment is a good example of that: I could get them asleep in a 1:7″ twist, 25-inch barrel, which was chambered in .22 PPC, but could not get them stabilized in a 20-inch 1:7″ .223 Rem. The answer always is to get a twist that’s correct.
These tips were adapted from Glen’s newest book, Top-Grade Ammo, available at Midsouth. To learn more about this book and other Zediker titles, and read a host of downloadable articles, visit ZedikerPublishing.com.
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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.
Share the post "Common Misconceptions about Twist Rate and Stabilization"
Here’s an extreme range of .224-Caliber bullets: 35gr varmint bullet and 90gr match bullet. Of course, along with bullet length/design, you need to consider MV when choosing twist rate.
Even with the same caliber (and same bullet weight), different bullet types may require different rates of spin to stabilize properly. The bullet’s initial spin rate (RPM) is a function of the bullet’s muzzle velocity and the spin imparted by the rifling in the barrel. You want to ensure your bullet is stable throughout flight. It is better to have too much spin than too little, according to many ballistics experts, including Bryan Litz of Applied Ballistics. Glen Zediker has some basic tips concerning barrel twist rates and bullet stability. These come from his latest book, Top Grade Ammo.
Choosing the Right Twist Rate
I’d always rather have a twist too fast than not fast enough. Generally… I recommend erring toward the faster side of a barrel twist decision. 1:8″ twist is becoming a “new standard” for .224 caliber, replacing 1:9″ in the process. The reason is that new bullets tend to be bigger rather than smaller. Don’t let a too-slow twist limit your capacity to [achieve] better long-range performance.
Base your next barrel twist rate decision on the longest, heaviest bullets you choose to use, and at the same time realize that the rate you choose will in turn limit your bullet choices. If the longest, heaviest bullet you’ll shoot (ever) is a 55-grain .224, then there’s honestly no reason not to use a 1:12″. Likewise true for .308-caliber: unless you’re going over 200-grain bullet weight, a 1:10″ will perform perfectly well.
Bullet Length is More Critical than Weight
Bullet length, not weight, [primarily] determines how much rotation is necessary for stability. Twist rate suggestions, though, are most usually given with respect to bullet weight, but that’s more of a generality for convenience’s sake, I think. The reason is that with the introduction of higher-ballistic-coefficient bullet designs, which are longer than conventional forms, it is easily possible to have two same-weight bullets that won’t both stabilize from the same twist rate.
Evidence of Instability
The tell-tale for an unstable (wobbling or tumbling) bullet is an oblong hole in the target paper, a “keyhole,” and that means the bullet contacted the target at some attitude other than nose-first.
Increasing Barrel Length Can Deliver More Velocity, But That May Still Not Provide Enough Stability if the Twist Rate Is Too Slow
Bullet speed and barrel length have an influence on bullet stability, and a higher muzzle velocity through a longer tube will bring on more effect from the twist, but it’s a little too edgy if a particular bullet stabilizes only when running maximum velocity.
My failed 90-grain .224 experiment is a good example of that: I could get them asleep in a 1:7″ twist, 25-inch barrel, which was chambered in .22 PPC, but could not get them stabilized in a 20-inch 1:7″ .223 Rem. The answer always is to get a twist that’s correct.
These tips were adapted from Glen’s newest book, Top-Grade Ammo, available at Midsouth. To learn more about this book and other Zediker titles, and read a host of downloadable articles, visit ZedikerPublishing.com.
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With the Berger Southwest Nationals underway this week, we thought we’d steer our readers to a very useful resource, courtesy Berger Bullets. This online Stability Calculator helps shooters determine the optimal twist rate for their choice of projectiles.
Berger Twist-Rate Stability Calculator
On the Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This cool tool tells you if your chosen bullet will really stabilize in your barrel.
How to Use Berger’s Twist Rate Calculator
Using the Twist Rate Calculator is simple. Just enter the bullet DIAMETER (e.g. .264), bullet WEIGHT (in grains), and bullet overall LENGTH (in inches). On its website, Berger conveniently provides this info for all its bullet types. For other brands, we suggest you weigh three examples of your chosen bullet, and also measure the length on three samples. Then use the average weight and length of the three. To calculate bullet stability, simply enter your bullet data (along with observed Muzzle Velocity, outside Temperature, and Altitude) and click “Calculate SG”. Try different twist rate numbers (and recalculate) until you get an SG value of 1.4 (or higher).
Gyroscopic Stability (SG) and Twist Rate
Berger’s Twist Rate Calculator provides a predicted stability value called “SG” (for “Gyroscopic Stability”). This indicates the Gyroscopic Stability applied to the bullet by spin. This number is derived from the basic equation: SG = (rigidity of the spinning mass)/(overturning aerodynamic torque).
If you have an SG under 1.0, your bullet is predicted not to stabilize. If you have between 1.0 and 1.1 SG, your bullet may or may not stabilize. If you have an SG greater than 1.1, your bullet should stabilize under optimal conditions, but stabilization might not be adequate when temperature, altitude, or other variables are less-than-optimal. That’s why Berger normally recommends at least 1.5 SG to get out of the “Marginal Stability” zone.
In his book Applied Ballistics For Long-Range Shooting, Bryan Litz (Berger Ballistician) recommends at least a 1.4 SG rating when selecting a barrel twist for a particular bullet. This gives you a safety margin for shooting under various conditions, such as higher or lower altitudes or temperatures.
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You may not realize it… but to get the optimum BC from your bullets (i.e. the lowest aerodynamic drag), you must spin the bullets fast enough. Bullet drag increases (as expressed by lower BC) if the bullet spins too slowly. Bryan Litz of Applied Ballistics explains how BC changes with twist rates…
More Spin, Less Drag
In this article, we look at how twist rate and stability affect the Ballistic Coefficient (BC) of a bullet. Again, this topic is covered in detail in the Modern Advancements book. Through our testing, we’ve learned that adequate spin-stabilization is important to achieving the best BC (and lowest drag). In other words, if you don’t spin your bullets fast enough (with sufficient twist rate), the BC of your bullets may be less than optimal. That means, in practical terms, that your bullets drop more quickly and deflect more in the wind (other factors being equal). Spin your bullets faster, and you can optimize your BC for best performance.
Any test that’s designed to study BC effects has to be carefully controlled in the sense that the variables are isolated. To this end, barrels were ordered from a single barrel smith, chambered and headspaced to the same rifle, with the only difference being the twist rate of the barrels. In this test, 3 pairs of barrels were used. In .224 caliber, 1:9” and 1:7” twist. In .243 caliber it was 1:10” and 1:8”, and in .30 caliber it was 1:12” and 1:10”. Other than the twist rates, each pair of barrels was identical in length, contour, and had similar round counts. Here is a barrel rack at the Applied Ballistics Lab:
Applied Ballistics used multiple barrels to study how twist rate affects BC.
“The Modern Advancements series is basically a journal of the ongoing R&D efforts of the Applied Ballistics Laboratory. The goal of the series is to share what we’re learning about ballistics so others can benefit.” –Bryan Litz
Barrel twist rate along with velocity, atmospherics, and bullet design all combine to result in a Gyroscopic Stability Factor (SG). It’s the SG that actually correlates to BC. The testing revealed that if you get SG above 1.5, the BC may improve slightly with faster twist (higher SG), but it’s very difficult to see. However, BC drops off very quickly for SGs below 1.5. This can be seen in the figure below from Modern Advancements in Long Range Shooting.
The chart shows that when the Gyroscopic Stability Factor (SG) is above 1.5, BC is mostly constant. But if SG falls below 1.5, BC drops off dramatically.
Note that the BC drops by about 3% for every 0.1 that SG falls below 1.5. The data supports a correlation coefficient of 0.87 for this relationship. That means the 3% per 0.1 unit of SG is an accurate trend, but isn’t necessarily exact for every scenario.
It’s a common assumption that if a shooter is seeing great groups and round holes, that he’s seeing the full potential BC of the bullets. These tests did not support that assumption. It’s quite common to shoot very tight groups and have round bullet holes while your BC is compromised by as much as 10% or more. This is probably the most practical and important take-away from this test.
To calculate the SG of your bullets in your rifle, visit the Berger Bullets online stability calculator. This FREE calculator will show you the SG of your bullets, as well as indicate if your BC will be compromised (and by how much) if the SG is below 1.5. With the stated twist rate of your barrel, if your selected bullet shows an SG of 1.5 (or less), the calculator will suggest alternate bullets that will fully stabilize in your rifle. This valuable online resource is based directly on live fire testing. You can use the SG Calculator for free on the web — you don’t need to download software.
Learn More About SG and BC This article is just a brief overview of the interrelated subjects of twist rate, Gyroscopic Stability, and BC. The coverage of twist rates in Modern Advancements in Long-Range Shooting is more detailed, with multiple live fire tests.
Other chapters in the book’s twist rate section include: · Stability and Drag – Supersonic
· Stability and Drag – Transonic
· Spin Rate Decay
· Effect of Twist rate on Precision
Other sections of the book include: Modern Rifles, Scopes, and Bullets as well as Advancements in Predictive Modeling. This book is sold through the Applied Ballistics online store. Modern Advancements in Long Range Shooting is also available in eBook format in the Amazon Kindle store.
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This is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting Vol. 1 is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.
Increasing Barrel Length Can Deliver More Velocity, But That May Still Not Provide Enough Stability if the Twist Rate Is Too Slow
