Barrel Twist rate and Bullet Stability – What you need to know

Understanding Twist – Bullet Stabilization by Sierra Bullets’ Ballistic Technician Paul Box, for the Sierra Bullets Blog. According to the questions we receive on a daily bases on our 800 line (Customer Service), twist is one of most misunderstood topics in the gun industry. Let’s take a closer look at this mystery to better understand what twist means. What does it mean when you see the terms 1:14′ (1-14) and 1:9′ twist? A rifle with a 1:14’’ twist means that the bullet will make one full rotation every fourteen inches along the barrel. A 1:9’s twist will turn one time for every nine inches of the barrel. Here’s a thing that some people struggle with. I’ve received calls from shooters who thought that a 1:14″ twist was faster than 1:9″ because the number on the 1:14″ was higher. The easiest way to remember is that the higher the number is, the slower the rate of twist. The biggest misconception is that if you have a.223 rifle with a 1” twist then your rifle will not stabilize a 55gr or lighter bullet. Let’s see what is needed. The faster the twist, the longer the bullet will be for its diameter. In the case of.223 with 1:8’’ twist, it was designed to stabilize bullets 80gr in this diameter. In reality, the opposite is true. A 1:8″ will spin a bullet faster than is necessary to stabilize a bullet of that length. If your bullet has good concentricity, then over-spinning will not [normally] affect its accuracy potential. [Editor’s note: Additionally, the faster twist will not, in most cases, decrease velocity. Testing done by Bryan Litz of Applied Ballistics Labs has confirmed this. There may be a minor speed loss. On their barrel blanks, many barrel makers mark the twist rate and bore dimension. Imagine your truck’s tires. You can drive your truck at 35 MPH with new tires that have been balanced properly at the tire shop. You can drive on the highway at 65 MPH, and they will still spin perfectly. A bullet behaves the same. Once I loaded some HP 35gr bullets into a 22-250 with a twist of 1:8″. After three shots, the average velocity of 4584 FPS was obtained with a RPM level 412,560. The group was.750″ at 100 yards. This is a great example of how it is difficult to over-stabilize good bullets. Erik Dahlberg’s illustration of twist-rate courtesy FireArmsID.com. Photo of a Krieger barrel courtesy GS Arizona.