With the welcome deluge of new gun owners, and especially the new rifle shooters, TTAG has been getting a lot of great questions. Some reoccurring themes include queries about the effects of barrel length and the twist rate of rifling.
Rifling a barrel is a pretty simple idea, and it’s been around for a long time, at least 500 years. The concept is simple, form a helical pattern in the barrel that’s slightly smaller than the projectile to be passed through it, and that twisting pattern will make the bullet spin in flight.
That spin is pretty important, as it greatly aids in gyroscopic stability. If you’d like to fully understand this concept, grab a basic physics book, or do an internet search for the terms “gyroscopic stability” and “conservation of angular momentum“. Those resources will explain to you something you likely already know though your own life experience: objects spinning tend to travel more in a straight line and seem to be less affected by outside forces.
Actually, making the rifling in a gun barrel can be done in a few ways. The most common is “button” rifling, where a roundish “button” with edges cut for grooves is pushed or pulled through a barrel to create the necessary pattern. There’s also “cut” rifling, which is the oldest method and is still the most often preferred by the best barrel makers in the world. It can also be done a few different ways, but the general notion is that a bit is passed through the barrel to scrape away bits of metal until the required pattern of lands is created.
There’s also hammer forging, where a barrel is hammered around a mandrel to create the helical pattern. Hammer forging is not common for high-end custom rifle barrels, but very common in pistol barrels and other mass-produced guns. They work just fine for this application.
That pattern of ridges and valleys (lands and grooves) created by rifling (the verb) the barrel is pretty important, and application-specific. There are many different shapes these lands can take, and different numbers of them. That matters some, and matters more and more depending on how specialized the application is. If you’d like to go down the rabbit hole on land shapes and patterns, know that it is a deep one, with many branches.
The most obviously important thing about the helical pattern inside that bore of a barrel is how tightly it twists. Called “twist rate,” it’s effectively how many times the rifling completes a full 360° revolution in a given length of barrel and, therefore, how quickly it spins the bullet (bullet RPM is a function of its velocity and the barrel’s twist rate).
Twist rates in the U.S. are measured as how many inches of barrel are required for the rifling to make one full rotation and are expressed as one turn per X inches of barrel. For instance, rifling that spins the bullet one full rotation per seven inches of rifling is a 1:7 twist. Likewise, 1:12 is one rotation per 12 inches, etc.
Yes, this means that a 6-inch barrel with 1:12 rifling will only spin the bullet a half rotation before it exits the muzzle. That’s okay. There’s absolutely zero requirement that a projectile complete any minimum or maximum number of spins while inside the barrel.
The vast majority of folks don’t know the twist rate of their firearm, and for the kind of shooting they do, they don’t need to. If you’re shooting your AR-15 at 100 yards at a 6-inch target, or you hunt within a couple of hundred yards with a .264 to .45 caliber rifle, you can just buy any commercially available ammunition advertised for hunting applications and you’ll be good to go.
If you’d like to be a bit more precise than that, or you have some specific applications in mind, then the twist rate of your barrel and the shape of your bullet are going to become much more important.
Quite simply, a longer, more needle-shaped object is harder to stabilize than a shorter, more ball-shaped object.
For my fellow flintlock and traditional cap and ball rifle shooters out there, this is particularly important, and has been well understood for almost two hundred years. My 18th century flinter has a 1:60 twist. For an approximately .50 caliber round ball, that works just fine. It will drill a 6″ circle at 100 yards all day with a wide range of powder loads.
But load that up with a longer conical bullet and the group size will quadruple. It doesn’t take much spin to help stabilize a round ball, but elongate the projectile at all, and you’ll need to double that rate of twist, or more.
For the modern shooter, there are some circumstances where knowing and understanding your twist rate is vital.
First, and most common, are those concerned with precision at longer ranges. The first iteration of the FN SCAR 20S has a 1:12 twist rate. For the bullets it was designed to shoot — the 168gr and 175gr Sierra Match King — that will stabilize the bullet just fine.
But there are other bullet shapes, like the 178gr Hornady ELD, that won’t spin fast enough to stabilize with that same twist rate under all atmospheric conditions. If you’re shooting very small targets, or just shooting from far away, that will make a big difference in how predictable the flight path of that bullet will be.
Folks shooting very heavy subsonic rifle projectiles — like the subsonic loads for the .300 Blackout — need to get a fast twist to stabilize those long bullets, but they may have an additional reason to get the rotation even faster than is required for stable flight. Some companies make copper rounds designed specifically to open up at subsonic velocities. These companies, like Discrete Ballistics, suggest that an extremely fast twist rate, like 1:5, helps to allow the projectile to fully open and then cut through tissue like a propeller.
So if a faster twist rate helps to stabilize a bullet, why aren’t all barrels made with the fastest twist rate possible? Well, for a few reasons.
The faster the twist rate the more resistance on the bullet. That just makes sense; as the bore is more perpendicular to the lands, there’s more resistance as the bullet travels the length of the barrel. This slows the bullet down a little, which has a slight negative effect not only on energy delivered, but predictability at long range. It also leads to slightly increased recoil and a slightly shorter barrel life. But again, only slightly.
There is also the possibility of bullets literally flying apart in flight. This is only a concern with fast spin rates on light, thin jacketed rounds moving extremely fast, like we see with the .22-250 Remington and other similar varmint calibers using varmint bullets.
A twist that is too fast may also have a negative effect on long-term precision as well, although not as much as too slow a twist. The long-held belief is that if a bullet is spinning too fast, then the bullet’s nose won’t tip down after the apex of its curve on the way to the target. This nose-up attitude creates increased drag.
This is technically true, but the amount of additional drag on a small arms bullet is so minute that even in the realm of precision shooting, where the tiniest variables count, this one really doesn’t.
The real issue is that a faster twist rate causes an increased dispersion. In this case, we can think of dispersion as group size. Bullets are imperfect things. As they aren’t truly perfectly balanced around their axis, the faster it twists, the more dispersion (bigger group size) will result.
This is predictable. Simply, if either the imbalance in the bullet is doubled, or the twist rate is doubled, then the dispersion doubles. A more complete explanation of this phenomenon can be found here, provided by Lija.
So twist rate matters, but what twist should you use? Fortunately, most of the time we can find that information pretty simply, by looking at the specifications of the rifle and the suggested twist rate that most bullet manufacturers publish. That doesn’t always work, and the more specific you get in your application, the less likely it is to work.
For instance, I needed to know if my preferred home cast lead 405 grain .459″ bullet would stabilize in the new Marlin 1895CB with it’s 1:20 twist rate. For that, I needed to use Greenhill’s Formula or the Miller Stability Rule. For this, you can do the math or just go to the JBM Ballistics free online calculator.
Some of you have noticed that some of the twist rates on some guns are slower than the barrel length. That 1:60 twist on my flinter? Well I don’t have a five foot long barrel. That’s especially true for pistol calibers, where the twist rate almost always means that the bullet doesn’t make a full rotation before leaving the barrel.
Remember your simple physics. An object in motion tends to stay in motion until acted upon by an outside force. Once that bullet starts spinning, it spins at that rate until something (like air) slows it down or stops it. That’s why a longer barrel doesn’t necessarily stabilize the bullet any better than a short one.
However, a longer barrel — to a point — results in a faster traveling bullet. That can lead to increased precision at long ranges, and definitely leads to increased energy delivered to the target.
The point at which barrel length increases the speed of the bullet depends on many factors. Bullet weight, diameter, composition, shape, powder burn rate, land geometry, twist rate, barrel material, and more. The conversion of the solid powder to hot gas propels a bullet with enough force that it overcomes the resistance provided by contact with the lands, but not forever.
For some cartridges, that force is enough to keep the bullet accelerating to beyond 30″ of barrel length. Some others, like the 500 S&W Magnum round fired from a Big Horn Armory Model 89, those bullets will slow after just 18″ of travel. Most .45ACP loads slow after 16″, but most .45 Colt loads are still gaining speed past 20″, as are most .223 Remington and 5.56NATO loads.
If there’s a way to comprehensively predict this point-of-no-return in velocity for any particular barrel and load, I don’t know it.
For most new shooters, it’s best to stick to the basics and start with the bullet. Decide what you want the bullet to do, decide on a bullet, and then decide on a firearm that fires that bullet, based on information provided by the ammunition and firearm manufacturers.
Barrel twist rate and length may be part of your decision-making process, but probably not in the beginning. Eventually it will, and the more specific your needs, the more carefully you’ll need to consider the twist rate and length of your barrel.