Barrel Twist Rate, Why it matters
Barrel Twist Rate? What Is It? Why Does It Matter?
If you ever find yourself looking at specs for a rifle (or some pistols), you may see a specification that says something like this: Twist 1:8
What is ‘twist’, and what does that have to do with the firearm’s performance?
History of Rifling & Bullet Design
To explain that we have to go back in history a little ways… The story starts with rifling, which are the spiral grooves found in the barrel of pretty much any firearm with the exception of smoothbore muzzle loaders (see image to the right). Originally, all guns were smooth bore barrels, firing round ball projectiles… with relatively poor accuracy and limited range. However, if the projectile is spun in a plane perpendicular to its flight, you can see big gains in distance and accuracy. This concept is actually a lot more familiar than you may think; everyone knows that to throw a football very far or very accurately, you need to throw it with a spiral… that spinning motion stabilizes its flight, due to gyroscopic forces. This effect is especially pronounced if the bullet is somewhat pointed on its end (again, similar to a football), due to aerodynamic forces on the projectile. The development of the modern shape of a bullet came about in 1847, with the advent of the “Minie ball” (named for the inventor, not the size of the projectile).
Though gun makers had been experimenting with cutting spiral grooves in barrels for many years, the combination of poor projectile design (round balls) and difficulty in manufacturing made rifling fairly uncommon up until just before the American Civil War. The invention of the Minie ball (which had a slightly hollow base, allowing the back edge of the bullet to expand upon firing, and grip the rifling grooves well) showed that the rifling concept was truly a game-changer in terms of performance. Suddenly, the effective range of long arms increased from about 100 yds up to nearly 1,000 yds… and both rifling and the general design of the Minie ball (shown to the left) were permanently set in firearms design.
What is Twist Rate?
So how does all of that relate to twist rate? Twist rate is simply an expression for how aggressively the barrel puts spin on the bullet. When you see it expressed as a ratio (as in the example at the beginning of this story), it means that the projectile makes one complete turn (360o) in a certain number of inches of barrel length. If the twist rate is 1:8, that means that the bullet turns one complete turn in eight inches of barrel length. This means that if the second number in the ratio is lower, the twist rate is actually faster (because the bullet made a complete turn in a shorter distance).
So, what is the ‘ideal’ twist rate? That’s a much more complicated question than it sounds…. generally, the goal of the gun maker is to provide sufficient spin to give good stability to the rounds that are expected to be fired, but not to ‘over spin’ the bullet. There are some rules of thumb, though, that help illustrate where the ‘sweet spot’ for spin rate is. Larger caliber projectiles generally do not need as fast of a spin rate as smaller diameter rounds to stay stable in flight; this is because the larger diameter of the round leads to greater gyroscopic inertia (so the spin rate can be less and still get the desired effect). Smaller diameter rounds therefore need faster spin rates. In general, pistol barrels have a slower spin (bigger second number on the twist rate) than rifles, as the short but generally wider pistol rounds don’t need to be spun as fast as the smaller diameter, but much longer rifle rounds. The weight of the bullet also plays a role in determining how aggressive the twist rate has to be, with heavier rounds generally needing more spin to stabilize (though the interactions between diameter, bullet length, and weight are complex, and beyond the scope of this article).
As an example of how this works, let’s look at the familiar AR-15 platform. Chambered in 5.56mm/0.223 cal (we’ll save the discussion for the similarities and differences between those for another article), the rifle can fire a range of bullet weights. Testing has shown that the best twist rate for common 55 grain ammunition is 1:9, while bullets between about 62 grains and 77 grains do best with a 1:8 twist. Heavier ammunition in this caliber can benefit with the very fast twist rate of 1:7. Military rifles in this caliber generally use a 1:7 twist. This is all summarized in a chart produced by the NRA, shown here:
So What Happens if the Twist Rate is Too High or Too Low?
By knowing the twist rate of your gun, you can pick ammunition that will shoot the most accurately with that spin rate. So what happens if you spin the bullet too fast or too slow? Is it dangerous?
Generally, no, over spinning or under spinning the bullet is not dangerous. Provided that the ammunition is the proper caliber (and of course, assuming the weapon is in good working order), there is no safety issue. However, under spinning the bullet (for example, using a heavy bullet in a gun with a low twist rate) may lead to the bullet yawing, and potentially even tumbling in flight; you can see this if you are shooting paper and see a ‘keyhole’ bullet entry mark on the paper. If the bullet is yawing, you’ll see an elongated, but still fairly round hole. If the bullet is tumbling, that elongation will be even greater. This will affect not only the entry mark on the paper, but also the accuracy will suffer greatly, especially at longer distances.
On the other hand, if the bullet is overspun (for example, from using a very light bullet with an aggressive twist rate on the barrel), the bullet can in theory spin so fast that it is torn apart and by gyroscopic forces and disintegrates in flight…though in practice, it’s pretty much impossible for this to happen with a FMJ (full metal jacket) bullet. However, a higher-than-needed twist rate can cause accuracy problems; any slight defects in manufacturing of the round (such as slight imbalance) will be accentuated by high spin rates, potentially causing yaw and drift problems.
With all that being said, if you want to get the best performance out of your firearm, twist rate and bullet weight are factors that need to be considered. It stands to reason that if you use the ammunition that your gun was designed for, you’ll get the best results!
