Practical Considerations On Twist Rate

Achieving bullet stability is a function of bullet length, velocity, and rate of spin. Higher copper content adds length. A 140-grain 7mm Trophy Bonded Bear Claw (left) is longer than a heavier 145-grain bullet of conventional construction.

There is a business and engineering tool called the "pareto analysis." In simple terms, it helps you identify the 20 percent of your efforts that contribute 80 percent of the benefits. In other words, find what you don't need to worry about. Very practical.

As I've spent more time shooting and reloading, I've learned to weed out some of the details I used to fret over so I can focus more time on what really matters.

Judging from phone calls I handled while working at CCI-Speer, one area that probably gets too much worry for its effects is rifling twist rate in hunting rifles. Taking the practical approach, the average hunter does not need to invest a lot of time researching twist rates. This is absolutely different for the benchrest or long-range shooter for whom "no group is too small," but the rest of us can usually rely on the gun companies to pick a decent twist rate.

If a big-game rifle can consistently print groups around 1 inch at 100 yards, there will seldom be a need for a special twist rate. You can take the money you could spend on a custom barrel and invest it in ammo or components to hone your shooting skills. Face it, most of us can use the practice.

Bullet Spinning 101
As you probably know, the spiral grooves that barrel makers so painstakingly cut into bores are the rifling. These capture and spin bullets at very high rotational velocities to produce artificial stability to overcome the natural stability, which is actually their tendency to randomly tumble. The accompanying chart lists the calculated rotational velocities at the muzzle for some common cartridges.

Anything propelled through air will strive to reach natural stability. This usually means flying with the center of gravity (CG) leading the way, like a spear. A round projectile has its CG in the middle. Should the round ball tip in flight, the surface area presented remains roughly the same, and the air resistance working to slow the bullet is relatively constant.

Changing to a cylinder presents a problem. It will tumble as it struggles to get the CG forward, and air resistance will vary with the bullet's attack angle. When the cylinder is sideways to the airflow, the resistance is very large compared to when its small end is forward, and this presents a terrible flight model. If you put a point on that cylinder, the CG shifts toward the other end, exacerbating the problem.

Artificial stability overcomes the natural tendency of flying objects to force the CG forward, allowing us to accurately shoot projectiles that could never fly well under natural stability. Achieving stability is a function of bullet length (not weight), velocity, and rate of spin. The ideal is to find some combination of these factors that is not close to the stable/unstable boundary. If you are close to the boundary of stability, an unexpected drop in velocity or changing to a bullet of the same weight but greater length can push stability over the edge. The result is the telltale mark on the target of an unstable bullet smacking the paper sideways: the dreaded "keyhole."

Understabilized Bullets
Keyholing is the biggest fear when considering twist rate. Too little artificial stability means that natural stability wants to resume its reign before the bullet travels a desired distance, and the bullet yaws, opening group size dramatically. That's a pretty obvious condition. However, when we approach the boundary conditions, more subtle effects can do a "gotcha" on you.

If you are running near the stable/unstable boundary, keyholing may be delayed or not show up as dramatically as expected. Over a certain range from the muzzle, the bullet may do what you want--strike nose-on and give you a nice group. As distance increases, its artificial stability begins to decay. Bullets may appear to strike nose-on but are actually slightly tipped. Depending on the bullet and how the paper target reacts to it, close examination of a bullet hole may show that the normal black "wipe ring" is not quite symmetrical. However, if you don't mind shooting more, there is a more revealing pattern.

A bullet with good artificial stability will print groups that are directly proportional to the firing distance over its useful range. Group size increases in a linear and predicable manner. A load that prints 1 inch at 100 yards can be expected to print around 2 inches at 200 and about 3 inches at 300, assuming you do your part. That's a linear growth of the group size, and that's what we want.

It's possible to be so close to the stability boundary that the bullet loses it over typical hunting ranges. If reading bullet holes make you suspicious about stability, shoot the same load at several distances and see how the group size grows. If you get 1 inch at 100, 4 inches at 200, and 10 inches at 300, something is dreadfully wrong. This nonlinear growth screams "too little stability." One of the stability factors we're considering--velocity, bullet length, or twist rate--is off.

Thirty years ago, it was hard to get into stability trouble from changing bullets--most .30-caliber 180-grain spitzers were about the same length regardless of brand. Today, the new designs that add more copper to bullets--or eliminate it entirely--mean we have bullets that are longer than the ones that were around when cartridges were developed and rifling twists were established. In 175-grain .28-caliber bullets, the all-copper Barnes X-Bullet is a full 20 percent longer than a similar Speer Grand Slam. Fortunately, .28-caliber rifles nearly all have twists faster than 1:10, so there is seldom a problem. However, the standard twist rate of the .308 Winchester is 1:12, so loading a longer bullet could put you close to the stability boundary.

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Calculated Rotational Muzzle Velocities

Cartridge Velocity (fps) Twist (inches per turn) Rotation Speed (rpm)
.22 LR12001654,000
.223 Rem.310012186,000
.223 Rem.31007318,857
.30-30 Win.235010169,200
.38 Spl.80018.7530,720
.223 WSSM390010280,800
.44 Mag.130018.7549,920
.30-06 Sprg.280010201,600
.308 Win.280012168,000
.45-cal. round ball15006616,364

In the case of the .308 Win., it's perfectly acceptable to go to a slightly lighter bullet to add velocity and therefore stability. Those longer-than-yesteryear's bullets are so much tougher than older designs that a lighter, modern bullet may well out-penetrate an older one that weighs more. An extra 100 fps gained from a lighter bullet may be all that's needed to keep the bullet nose-on way out there.

Overstabilized Bullets
This is the side of this subject that inspired me to write about it. The typical phone call went something like, "I have a .315 Big-Ouch Magnum, and at beer call last night, my buds said the 1-in-10 twist rate is too much!"

First, let's look at truths about what overly fast rifling twists can do:
1. Varmint bullets with thin jackets can literally rip apart in flight if the twist rate is too fast.
2. The effects of a flaw within the bullet are magnified and groups open.
3. The bullet "goes to sleep" farther from the muzzle.

The first seldom entered the equation until some rifle makers changed the twist in their .223 Remington rifles from the original 1:12 to the 1:7 that the U.S. military adopted for the 5.56mm NATO round. Varmint bullets' prime attribute--the thin jacket--simply could not stand up to a 40 to 50 percent increase in rotational forces. We're talking big-game rifles, so this argument doesn't usually apply to these more robust bullets.

The second truth is real, but those defects it requires are getting very rare as bullet makers continually improve manufacturing processes. With modern technology, bullets are incredibly concentric, and high concentricity negates adverse rotational effects.

The third relates to a known characteristic of spin-stabilized bullets--they "wobble" slightly as they leave the tight confinement of the rifled bore. Spin eventually corrects the wobble, usually in the first 50 to 80 yards, and the bullet settles into stable flight, or informally, it "goes to sleep." However, long bullets fired at high velocities from fast-twist barrels may not go to sleep until more than 100 yards from the muzzle.

This explains why some very experienced riflemen say that the 7mm Shooting Times Westerner often shoots similar group sizes at both 100 and 200 yards. At 100 yards, the bullet still has a little wobble that is not quite damped by the spin.

So will the longer-to-sleep effect make you miss game? Not if your rifle still shoots "minute of deer" at the ranges you expect to take a shot.

There is a more basic conception that too fast a spin will always increase group size when no other factors are at play. Like other shooting conceptions, I find that this one likely goes back to military experience, in this case, artillery. One of my fellow engineers at Speer was a former U.S. Army artillery officer, and he was very aware of twist-rate factors; one clearly revealed the source of the "too much twist" misconception.

In artillery, high-angle fire is the standard firing model. Some field pieces like howitzers may fire with the tube elevated to 40 to 70 degrees. For decades, the "fuse-in-nose" design ruled nearly all projectiles for land-based artillery, requiring contact detonation. The projectile had to hit the ground fuse-first or the main explosive charge may not ignite.

High-angle artillery fire required critical twist-rate considerations so that the projectile would turn nose-down after reaching the top of its very high arched path. It turns out that too fast a twist rate left the projectile spinning like a top at the peak of the trajectory where its vertical velocity approached zero. Typical of gyroscopic stabilization, the shell tried to stay as it was when fired: nose higher than base. As gravity moved into the driver's seat and the projectile started down, its nose tended to stay above the trajectory line instead of leading the way with the all-important fuse ready to hit first.

Every red-blooded "cannon cocker" expects a mighty blast when his handiwork arrives. The problem with a grossly overstabilized shell was that it very likely landed on its side or base, and the contact fuse would fail to initiate that big, satisfying "BOOM." If you had a huge paper target spread flat on the ground where the shell hit, you'd see a keyhole.

Let's get practical again. There are few if any sport-shooting conditions that qualify as high-angle fire to the

artilleryman. Even shooting a .45-70 cast bullet at a 1,000-yard target is technically direct fire. The elevation is only a few degrees compared to the whopping angles used in howitzers. As long as you are not shooting thin-jacket bullets and can deal with a bullet "sleeping" farther out than usual, don't get too hung up on the "perfect twist" for a big-game rifle.

Being Practical
You should consider new bullet developments when buying or building a new rifle. These have made some bullets longer for a given weight. A perfect example is the .22-250 Remington. Its standard 1:14 twist will handle standard bullets up to 55 grains and stubby bullets up to 70 grains if you can keep the speed up. However, nice pointy bullets heavier than 55 grains may need a faster twist to avoid stability issues. Some new .22-250 rifles are fitted with 12-inch-twist barrels, and you may wish to consider them over one with the "standard" 14- inch twist if the newer and heavier .22-caliber bullets appeal to you.

Beyond that, simply be aware that changing to a longer, high-tech bullet could push you close to the stability boundary. Realistically, the only modern cartridge where I foresee a problem is the .308 Win., where some rifles have 1:12-twist barrels. If you are shooting 180-grain conventional spitzers and change to an all-copper version, you may learn more than you want about stability problems. If this is a concern, verify the twist before buying; some companies fit 10-inch-twist barrels to their .308 rifles. Most rifle makers' websites have reference sections that let you find their standard twists.

Remember that we are not talking about huge changes in twist rate. Most tinkering would involve changing the twist by increments of only 1 to 2 inches. Obviously, there are likely some very negative effects to halving the twist rate, like going from 1:12 to 1:6, but rates under 1:8 are rare except in special applications--not big-game hunting.

This is about the practical aspects of big-game rifles. That means putting meat in the freezer or a fine mount in the den. It's totally different in target shooting, especially the long-range competitions. There, a twist-rate change of a mere half-inch may make the difference between taking home a blue ribbon and hanging out with the "also-ran" pack.

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