The Effect of Barrel Twist Rates on Muzzle Velocities
We will be interviewing Bryan Litz of Applied Ballistics tomorrow at SHOT Show in Las Vegas. As a sneak preview of some of the topics we’ll cover, here are some highlights of some important, original research conducted by Bryan and his Applied Ballistics team. Bryan wanted to know how much velocity was altered by twist rate. The “real world” test results may surprise you….
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. That book includes many 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?
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. 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 the testing in all 6 barrels from 1:8” to 1:12”. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the 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 a surprising number — much less than most folks would predict.] 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 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 in eBook format in the Amazon Kindle store.
Similar Posts:
- How Barrel Twist Rates Can Affect Muzzle Velocities
- How Muzzle Velocity Changes with Different Barrel Twist Rates
- Does Barrel Twist Rate Affect Muzzle Velocity? (Litz Test)
- How Bullet Velocity Is Affected by Barrel Twist Rate — Litz Test
- New Spin on BC — How Barrel Twist Rates Affect Bullet Drag
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Tags: Applied Ballistics, Barrel Twist Rate, Bryan Litz, Chronograph, Modern Advancements, rifling, Velocity
Would rather see data on one bullet weight fired from different twist rates at different velocities to determine bullet frangiblity in ballistic jell. Example a 22 caliber 60 grain hollow point fired from a 1-14 twist at 3600 fps versus same bullet fired from a 1-9 twist at 2900 fps. Bullets react differently on live targets based on rotational speed more so that velocity. IMO….
So velocity loss is basically a non event. Good to known.
I like Eddie’s comment. I have a 1/8 twist cal cal barrel in the cupboard that I do not know what to do with and keep on thinking a 308 Win would be good instead of another Whisper.
But contemporary window is that an 8 twist barrel will shake the core out and make the bullet more unstable at long ranges.
Velocity is one thing, but accuracy counts. Any comments on that aspect?
You should be fine with a 1-8 twist 308 Win Richard. The only real concerns with a fast twist is if the jackets will hold up but that’s usually not an issue unless running thin jacketed projectiles at high velocities. And there will likely be a little more spin drift as well, but other than those two things, personally I see no real reason not to do it up… Just my take on that though, have a nice day!
I known its a velocity study not a accuracy one but I cant help but wonder witch twist shot the best group.
Thanks Adam. I’m not sure why it would not run 155 – 175s without problems. Make it a finished length of 24 inches and see how a fast twist runs. Can always cut it shorter then.
Anytime Richard, I’m thinking of going 1-7 twist on my 7mm Rem Mag myself. Spin that 308 up and let her fly!
I’d like to see them have a strain gauge on each barrel and seen if there were any significant pressure differences with starting at a fast twist vs starting at a slow twist.
I’m with Eric. I think the issue is the ability to run higher velocity in a slower twist, due to lower pressures and not whether the same ammo is substantially quicker or slower in a tighter twist.Still interesting I guess, but missing the real point of the twist selection discussion a bit I think.
Hi
my question is how do you match a specific projectile to a specific twist to achieve the best accuracy.
Or what is the best velocity in what twist rate?
Thanks
Ed