Tyler Kee asked (paraphrasing here):
So what’s all this “barrel twist” hoopajoop? Does barrel twist impact my accuracy? And if so, what twist is best?
Continuing with my “AR for Dummies” kick, I figured this would be a great opportunity to discuss what is probably the most confusing part of buying an AR-15 and try to make it so simple that even Tyler will understand it. So let’s begin . . .
Modern firearms use a series of grooves inside the barrel called “rifling” to spin the projectiles (bullets) as they pass through the barrel. The bullets are sized to fit (snugly) down the barrel without the raised portions so when the bullet is pushed down a barrel with rifling those spiral designs cut into the side of the bullet and grab hold, forcing it to spin. We express the degree of rifling twist in a barrel by a ratio that describes the number of inches it takes to do one complete revolution (for example, 1:12 means 1 revolution in 12 inches).
There are two main reasons for why rifling is important, and both have to do with accuracy.
First, bullets are imperfect projectiles. No matter how well you make them, one side is always going to have more drag or a little more mass, and that imperfection will drag the flight path of the bullet in one direction or the other. Spinning the bullet forces the imperfections to be applied evenly to each direction, cancelling them out. Mostly.
Second, bullets in the real world don’t operate in a frictionless plane like they do in physics books. There are external factors by the boatload that impact where the round will hit, and spinning the bullet helps reduce the effect of those factors. The reason is that the spinning turns the bullet into a gyroscope, and as we students of physics know a gyroscope likes to stay oriented in a single direction and will resist changes. Changes that result from things like wind, for example.
So, one would assume that the faster you can get the round spinning the happier you’ll be. And, for the most part, you’re absolutely correct. The issue, however, is that if you spin a high velocity round (like the 5.56x45mm NATO) too much it has a tendency to disintegrate when it slams into the wall of air past the muzzle and no longer has the barrel to keep it together.
The other end of the spectrum is also no bueno. Bullets need to be spinning sufficiently fast to get that gyroscopic stabilization going (ever notice how gyroscopes will topple over after they slow down, even if the wheel is still spinning?), and if they aren’t properly stabilized they will begin to tumble. And tumbling is bad. The military ran into this problem when they tried using 1:14 barrels, and they’ve since spun things up quite a bit.
So the trick is getting the right level of spin on a given round, and thanks to decades of testing we pretty much know what works and what doesn’t.
As a rule of thumb:
- 40gr likes 1:12
- 55gr likes 1:9
- 77gr likes 1:7
Some people think that twist rate is a direct factor of bullet weight. Some people would also be wrong. Thanks to physics, we know that the ideal twist rate for a projectile is determined by the combination of the length of the projectile and the velocity, and not the weight of the projectile. But since we’re dealing with identical materials (lead, copper…) in a fixed diameter (.224 inches) and bullets are usually sold by the weight, we can use weight to determine which bullet is the right size for the rifling we have. Or, alternatively, we can determine what rifling we need based on bullet size.
The “rule of thumb” formula for twist rate, developed by George Greenhill, is:
Twist = (C x [Bullet Diameter in Inches Squared]) / [Length of Bullet in Inches]
Where “C” is 150 when the velocity is under 2,800 feet per second, and 180 when above. There’s some extra bits that are thrown in when you don’t have a lead core projectile, but that’s the basics. This formula can be used to either figure out the ideal twist for a given projectile, or figure out what the ideal length of projectile for a given twist is.
So what is the “right” twist rate to get? What is best? Well, that depends.
If you’re the average shooter and you own an AR-15 for general shooting and/or home defense purposes, you’re probably going to be running either 55gr or 62gr rounds through it. That’s the standard weight for most bulk ammunition (thanks to the insatiable appetite of the military for it), and that stuff is best stabilized by a 1:9 twist barrel. Most “standard” AR-15 rifles come in 1:9 by default.
If you plan on doing some prairie dog hunting and need light, zippy rounds then a 1:12 is probably best instead. It will still stabilize some of the heavier stuff (like 55gr) to a degree, but not for very far.
If you’re on the other end of the spectrum and long range accuracy is your kind of thing, then 1:7 is the twist rate to go with. Heavier (and therefore longer) bullets are more resistant to external factors and have more mass, which means they maintain velocity longer and are accurate at longer ranges. In other words, ideal for long distance shots. 1:7 will stabilize the biggest bullets you can cram in the case and do it well, but it will also stabilize the lighter stuff (like 55gr) out to (in my experience, at least) 300 – 400 yards.
This video is me running my 5.56 competition rifle for the first time since the Crimson Trace match (where I was running 40th out of 105 or so until I DQ’ed myself, which is why I was a touch cautious this time), and at about 3:55 you can see me hitting targets from 250 to 500 yards using 55gr ammo out of a 1:7 twist Noveske barrel. The 500 yard stuff didn’t go so well, but I still got a hit. Eventually. Point being that 55gr WILL stabilize (mostly) in a 1:7 barrel.
So what’s the answer? For most people, 1:9 is perfect. It will shoot the widest range of bullets, and will be especially good with the cheapest ammunition.