Bullet Weight and Twist Rate
by Greg Ritchie
You hear it all too often. What twist rate do I need for caliber X? Oftentimes the question is answered by another question. What bullet weight are you going to shoot? Stop right there! You see, it's not the weight of the bullet that determines what twist rate you need you need, but rather the length and velocity of the bullet that determines the twist rate.
Wait a minute you say. What about this chart that shows that twist rate X will stabilize bullets that weigh from Y grains to Z grains? You are saying it's wrong? It has always worked for me? Yes, I'm saying it's wrong. It should say twist rate X will stabilize bullets from Y length to Z length. The reason it has worked for you is that you have been using conventional lead core bullets. A .308 150 grain jacked lead core bullet will only be so long. But what if I am shooting a monolithic bullet, say an all copper bullet. It will be longer. Steel will be longer still.
As an example, let's take a couple of rifles I own and a couple of same weight bullets I use. The rifles are chambered in .223 Remington. Both have 1:9 twist barrels. Rifle #1 has a 16" barrel, rifle #2 has a 22" barrel. The bullets are both 62 grain and are .224 of an inch in diameter. One is conventional lead core, the other is a copper solid.
Rifle #1 shoots the lead core bullet just fine. The solid copper bullet? Not so much. The copper bullet often shows elongated holes in the target indicating bullet yaw. Rifle #2 on the other hand, shoots both bullets well.
According to one of the popular twist rate charts, the 1:9 twist is optimum for bullets weighing between 40 and 62 grains. What's going on here. Let's look a little deeper.
In the 1870's a British mathematician came up with what's called the Greenhill Formula, a simple math equation to determine optimum rifling twist for artillery pieces. This formula is still in use today. The formula looks like this:
C= Constant, use 150 for velocities below 2800 fps, 180 for velocities over 2800fps
D= Diameter in inches
L= Length in inches
The lead core bullet has a length of .797. The copper bullet has a length of .942. Out of the 16" barrel of rifle #1 the velocity is less than 2800 fps, so we use a constant of 150. Doing some quick math, we see that out of rifle #1, the optimum twist for the 62 grain lead core bullet is 1:9 and the 62 grain copper bullet is 1:8., No wonder the copper bullet does not do so well out of this rifle.
Let's look at rifle #2. Out of the 22" barrel this bullet exceeds 2800 fps, so we will use the constant of 180. A bit of quick math and we see the optimum twist for the bullets out of rifle #2 are, 1:11 for the lead core bullet and 1:9 for the copper bullet.
In summary, bullet weight is not an accurate way to determine the proper barrel twist and only works if comparing bullets constructed of similar materials.