December 6th 2022
Many barrel-makers mark bore dimensions and twist rate on their blanks. Does muzzle velocity change with faster barrel twist rates? Yes, but not as much as you might think. Although bullet speed is not affected by faster twist rates, it does slow down bullets a little. Bartlein.308 Win barrels with identical length and contour were tested. A 1:12?-twist barrel was 8 fps slower than a 1:8??-twist. 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 multi-barrel test is featured in Modern Advancements in Long Range Shooting Vol. 1. The book also includes fascinating field tests and a complete chronograph comparison.
What Tests Show: Barrel Twist Rate and Velocity
Bryan Litz
It is common belief that barrel twist rates will affect muzzle velocity. It is believed that a faster twist rate will resist forward movement of the bullet and slow it. Anecdotal evidence supports this idea, such as when someone replaces one barrel/twist with another brand and twist and observes an alteration in muzzle velocity. How do you know that twist rate is what affects muzzle velocity, and not barrel finish or bore/groove dimensions. Did you use the same chronograph for measuring velocity from both barrels with the same chronograph? Summary of Test Results
After all the smoke had cleared, we discovered that muzzle velocity was correlated to twist rate at an average rate of 1.33 FPS per inch. Your velocity will decrease by approximately 5 FPS if your twist rate is increased from a 1:12 to a 1:8. Turn to a 1/8? twist. — Bryan LitzSavage Test Rifle With Six Bartlein Barrels
Many shooters don’t have the equipment necessary to fully explore such questions. These are the exact types of things that we discuss in Modern Advancements in Long Range Shooting Vol. 1. We present experiments from the Applied Ballistics laboratory in that book. Some of these experiments were “Myth Buster” in tone, as we tried to verify (or deny!) popular preconceptions. Here’s an example of how we approached the question about barrel twist and muzzle velocity. Six.308 Win Barrels shot from the same rifle.
Six (6) barrels were purchased from the same manufacturer (Bartlein), each with the same length and contour and all chambered using the same reamer (SAAMI specification.308 Winchester). All six barrels were fitted with the same Savage Precision Target action and fired from the identical stock and bench setup. Common ammunition was fired from all six barrels with different twist rates and rifling configurations. This allows us to compare the effect of twist rate on muzzle velocity with reasonable confidence. An energy balance is used to predict how much velocity you’ll lose if your bullet has a faster twist. The.30 caliber 175 gr bullets will lose 1.25 fps for every inch-unit barrel twist (e.g. A 1:8? The twist is expected to be 1.25 fps faster than a 1:9? twist).
Below is data showing the relationship between Twist Ratio and Muzzle Velocity (MV), for different barrel twist rates and rifling types. The three 1:10? barrels are fast and slow. The three 1:10? twist barrels were: 5R (canted Land), 5 Groove, and 5 Groove left hand twist. We then tested all six barrels with twist rates ranging from 1:8 to 1:12? We tested all 6 barrels, with twist rates ranging from 1:8? to 1:12?. After all the smoke cleared out, we discovered that muzzle velocity correlates with twist rate at an average rate of 1.33 fps for every inch of twist. Your velocity will decrease by approximately 5 fps when you change from a 1:12 to a 1:8. Turn to a 1/8? twist. [Editor] This is an average of all lengths tested. The actual variance is between 1:12 and 1:8? The actual variance between 1:12? und 1:8? Here was 8 FPS. The math prediction was close in this case, but we must remember that live fire results can be unpredictable. Uncertainty can be viewed in terms of the conclusions that the results can support with confidence. This is a brief summary of one test case. Modern Advancements in Long-Range Shooting Vol. 2 covers twist rates. 1 is more detailed and includes multiple live fire tests. The results can be extrapolated to other calibers or bullet weights. The question of “how twist rate affects the muzzle velocity” is answered in full.
? Supersonic Stability and Drag
? Transonic: Stability and Drag
? Spin Rate Decay
? Effect of Twist rate upon Precision. Other sections include Modern Rifles and Scopes, Bullets, and Advancements in Predictive Modeling. The Applied Ballistics online shop sells this book. Modern Advancements in Long Range Shooting are also available in eBook format on Amazon Kindle.
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