September 22nd, 2020Does muzzle velocity change when barrel twist rates are increased or decreased? Yes, but not as much as you might think. The bullets are slowed down by faster twist rates, but it is not a significant loss. A 1:12?-twist Bartlein barrel of identical length and shape was only 8 fps quicker than a barrel with 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, unique test is featured in Modern Advancements in Long Range Shooting Vol. 1. This book contains other fascinating field tests including a chronograph comparison.
What tests reveal about barrel twist rate vs. velocity
Bryan Litz
It’s a widely held belief that barrel twist rates are important because they reduce muzzle velocity. The idea is that a faster twist rate will slow down the bullet’s forward motion. Some people have reported this anecdotally, for example when they replace a barrel with a twist or brand that is different from the original and notice a difference in muzzle velocity. How do you know that the twist rate was what affected muzzle speed and not the barrel finish or bore/groove dimension? Did you use the exact same chronograph for both barrels to measure velocity? Do you trust your chronograph? Summary of Test Results
After all the smoke had cleared, we found a correlation between muzzle velocity and twist rate. The average rate was approximately 1.33 FPS for every inch of twist. If you go from a 1:12 to a 1:8, your velocity will be reduced by approximately 5 FPS. If you go from a 1:12? twist. Bryan LitzSavage test rifle with six Bartlein Barrels
Most shooters lack the equipment necessary to fully explore these questions. Modern Advances in Long Range Shooting Vol. 2 examines exactly these kinds of questions. 1. In that book, experiments conducted in the Applied Ballistics laboratory are presented. Some of these experiments were “Myth-Buster” style, as we tried to confirm or deny popular preconceptions. Here’s an example of how we addressed the question of muzzle velocity and barrel twist.
We purchased six (6) barrels, all of the same length, contour and chambering (SAAMI spec..308 Winchester), from the same manufacturer. All barrels were fitted onto the same Savage Precision Target and fired with the same stock and bench setup. All six barrels were fired with the same ammo, despite their twist rates and rifling. We can compare the effect of twist rate on muzzle velocity in a way that is both accurate and reliable. Before the live fire tests, we did the physics. In this case, a physics-based energy balance is used to predict how much velocity will be lost for a bullet with a little bit more rotational energy. The math predicts that the.30 caliber bullets with 175 grain will lose 1.25 fps for every inch-unit barrel twist (e.g. A 1:8? A 1:8? twist).
The data above shows the relationship between Twist rate and Muzzle Velocity for different barrel twist rates and types of rifling. The three 1:10? twist barrels are arranged from fast to slow. The barrels with a twist rate of 1:10? We tested all 6 barrels with twist rates ranging from 1:8? After the smoke cleared, it was determined that muzzle velocity correlates with twist rate at an average rate of 1.33 fps for every inch of twist. If you go from a 1:12 to a 1:8, your velocity will be reduced by approximately 5 fps. If you go from a 1:12? twist. [Editor: That is the average of all the lengths that were tested. What is the actual difference between 1:12 and 1:8? The actual difference between 1:12? Here was 8 FPS.] The math prediction was very close in this case, but we must remember that live fire results are always uncertain. Uncertainty must be considered when determining what conclusions can be made with confidence based on the test results. This is a short summary of a single case. Modern Advancements in Long-Range Shooting Vol. 1 covers twist rates in more detail. The coverage of twist rates in Modern Advancements in Long-Range Shooting Vol. The results are extrapolated to other calibers, bullet weights, and bullet types. The book’s twist-rate section includes chapters on:
? Stability and drag — Supersonic
? Stability and Drag – Transonic
? Spin Rate Decay
? The effect of twist rate on precision. Other sections of the book are: Modern Rifles and Scopes; Bullets; and Advancements in Predictive Modeling. This book can be purchased through the Applied Ballistics Online Store. Modern Advancements in Long Range Shooting can also be purchased as an eBook in Amazon Kindle.
Tags: Applied Ballistics, Barrel twist rate, Bryan Litz, Modern Advancements Book, Velocity Test
