Barrel Twist Rate and Bullet Weight

Do They Matter, and if so, Why?

Whether you’re shooting mass-produced ammunition out of a factory rifle or you’re hand-loading for a custom wildcat rifle, your barrel twist rate will have a major impact on the bullet weight class that your gun will shoot most consistently.

So, what’s twist rate, anyway? It refers to how quickly the rifling makes a full rotation down the bore. Twist rate is typically expressed as a ratio, such as 1:9 (one full rotation every nine inches) or 1:7 (one full rotation every seven inches.)

The relationship between twist rate and bullet weight is crucial for accuracy and consistency.

This twist rate works in conjunction with the bullet's weight and – more importantly – the length of the projectile to achieve proper stabilization (gyroscopic stability) during flight. Without proper stabilization, accuracy suffers.

To help visualize how this works, think about a football. When an NFL quarterback throws a 60-yard pass, it usually goes where he intended. As you watch the ball in flight, it spins in a tight, fast spiral. That precision comes from years of experience. He knows exactly how much spin to apply for proper stability, but he does it without thinking.

If he threw the ball with too little spin, it would wobble (also called “yaw”) and lose accuracy and velocity. On the other hand, if he spun a shorter, lighter Nerf ball at the same speed, it would over-stabilize, wobble, and possibly tumble mid-flight. The same principle applies to rifle bullets. The right twist rate ensures stable, accurate flight, while a mismatch leads to erratic performance.

Understanding barrel twist rate and bullet size/length

The twist rate you need depends on the bullet's length, which generally correlates with its weight. Faster twist rates (like 1:7) stabilize longer, heavier bullets, while slower twist rates (like 1:12) are better suited for shorter, lighter bullets.

Longer bullets require a faster twist (faster revolutions) to maintain a stable trajectory.

Some bullet and ammunition manufacturers are beginning to put barrel twist recommendations on product packaging to help consumers make informed decisions.

 

Here’s a quick reference table, though these are just guidelines:

Caliber	Bullet Weight (gr)	Recommended Twist Rate
.223 Rem	55gr	1:9 or 1:12
.223 Rem	77gr	1:7 or 1:8
.308 Win	168gr	1:12
.308 Win	180gr	1:10

 

The caveat here is copper solid (or monolithic) bullets. Because copper is lighter than lead, solid copper bullets will be lighter for their length than lead-core projectiles. If you want to get into the weeds, the ratio of weight to length is called “sectional density.”

As with all bullets, testing monolithic bullets in your rifle is the ultimate indicator of whether they’ll shoot well or not, but expect copper solids to behave similarly to lead bullets of comparable length, not weight.

 

When a bullet is not stabilized correctly, the performance of the rifle/ammo combination suffers.

• Understabilization: Occurs when a twist rate is too slow for the bullet. Symptoms include keyholing (bullets hit the target sideways) and poor groupings.
  
  
• Overstabilization: Happens when a twist rate is too fast. In most practical shooting scenarios, this isn’t as detrimental, but it can cause excessive drag or minor accuracy degradation over long distances.

 

Application

Different shooting disciplines call for different bullet and twist rate combinations. For example, some fox and coyote hunters who don’t frequently shoot beyond 400 years, but are shooting small targets that are very mobile may prefer the fastest bullets they can find. Light, high-velocity bullets drop very little in the first few hundred rounds, allowing the hunter to shoot without re-ranging their target if it disappears from view and reappears elsewhere nearby. Light bullets are flat until reaching extended ranges, where their trajectory drops off rapidly.

Many of today’s western hunters and long-range competition shooters are gravitating toward heavier, high-BC (ballistic coefficient) bullets and faster twist rates. Competitions like PRS and NRL Hunter have been at least partially responsible for driving the development of higher-BC projectiles.

This is precisely the reason the 7mm PRC is stealing so much attention from cartridges like the 7mm Remington Magnum. The Remington has roughly the same case capacity as the PRC, but the Remington case is longer, making it ill-suited for 180 or 190 grain projectiles. Hunters and shooters are looking at ballistic charts and finding better long-range performance with longer, slower bullets.

Let’s take a closer look at the 7mm comparison. Using the specs provided on the box (yes, box speeds are usually generous estimates), how does the 7mm Remington Magnum stack up against the 7 PRC?

For the following example, we’ll assume:

• Altitude: 4600
• Temp: 36°F
• Humidity: 50%
• Pressure: 25.26
• Wind: 10 MPH from 90° (3 O’clock)
• Zero range: 200 yards

7 PRC: Federal Premium loaded with 175 grain Hornady ELDX @ 3,000 FPS.

• G1 BC: .689
• Twist rate: 1:8

At 800 yards, the 7mm PRC requires a 13.42 MOA elevation correction (bullet drop) and a total windage correction of 2.87 MOA.

7 Rem Mag: Federal Power-Shok loaded with 150 grain jacketed soft point @ 3,110 FPS.

• G1 BC: .408
• Twist rate: 1:10

At 800 yards, the 7mm Rem Mag requires a 16.20 MOA elevation correction (bullet drop) and a total windage correction of 5.61 MOA.

That means that the 7mm Rem Mag in this example dops 2.72 MOA (22.87”) more and deflects 2.74 MOA (22.95”) farther than the 7 PRC at 800 yards. These numbers were generated with estimated muzzle velocities, but they illustrate the point.

Yes, the lighter bullet leaves the muzzle much faster, but the longer, heavier, higher BC bullet retains energy longer and deflects wind better. That means that the margin for error while finding a range or calling wind is substantially better with the longer bullet. The longer the distance to the target and the faster the wind, the greater the 7 PRC’s advantage.

Of course, many hunters have no need to shoot beyond, say, 200 yards. So, this may be a moot point. And there are certainly ethical considerations when taking live game at long distance.

Practical considerations

Matching a rifle's twist rate to bullet weight (and length) is fundamental for accuracy. While general guidelines exist, every rifle is unique, and finding the perfect combination often requires some experimentation.

If you’re a handloader, you’ll have no shortage of projectile options. If you shoot factory ammunition, try several different loads to see which proves best. Before you buy the ammo, though, find out what your twist rate is and shop accordingly.

If you don’t know your barrel twist rate, call the rifle manufacturer. Manufacturers can typically provide the twist rate when the serial number is known.

If you’re building a custom rifle, select a bullet weight class and specify the barrel twist rate accordingly. It’s good practice to have your gunsmith engrave the twist rate on the shank, next to the name of the cartridge the rifle is chambered for.

And finally, write the twist rate down in your file log. Oh, you don’t keep a rifle log? Read this:

https://www.agcomposites.com/media/19/maintaining-a-rifle-logbook