What is Rotational Velocity and Why Does It Matter?

Nick Shafer, Chief Engineer at the gun company Q, spends a whole lot of time thinking about how fast bullets spin. It’s one of the reasons why I wanted to ask him about rotational velocity and why shooters and hunters should care about it. I first heard about the concept a while ago, but on a recent visit to Q, I got to learn a lot more about it.

Up there in New Hampshire, Shafer and his team have been looking into how they can squeeze the most performance out of subsonic ammunition, short-barreled rifles, and suppressors. Specifically, he’s worked a lot on the 8.6 and 300 Blackout.

By using rotational velocity to his advantage, Shafer has been able to use a dead-quiet rifle with just an 8-inch barrel to take African game animals the size of elk with one-shot kills out past 200 yards. Here’s how it all works and why rotational velocity is something hunters should start thinking about.

First, What is Rotational Velocity?

When you’re talking about bullets, rotational velocity is how fast a bullet spins. It’s measured in Rotations Per Minute or RPM, just like the RPMs of a car. The speed at which a bullet spins is determined by the bullet's velocity and the barrel's twist rate. “For example, a 300 Win Mag in a standard 1-in-10-inch barrel going at a standard velocity on the SAAMI chart is doing 221,000 RPM,” Shafer says. The faster the twist rate and higher the velocity, the faster the bullet will spin.

How Rotational Velocity Can Be Manipulated

For years, rifle and ammunition manufacturers have used rotational velocity as a metric to get an idea of how stable a bullet will be in flight. The established sweet spot has always been between 200,000 to 300,000 RPM for most cartridges. Too slow, and the bullets become “understable” and inaccurate. Too fast, and you’ll exaggerate any imperfections in a bullet, also making it unstable. Worse yet, poor cup-and-core bullet designs can come apart at faster twist rates/higher rotational velocities.

Q likes a very fast twist rate for a few reasons. They make suppressors and specialized guns to shoot subsonic cartridges—and heavy, subsonic bullets don’t work well without fast twists. “Heavy bullets need to be stabilized with faster twists,” Shafer says. “For example, with an 8.6 Blackout subsonic round, in a 1-in-3 twist at 1000 feet per second, we’re at about 240,000 RPM, which is fairly comparable to a cartridge running at supersonic velocities from a slower-twist barrel.”

But, according to Shafer, things start to get interesting when you switch to supersonic rounds out of those same fast-twisted barrels. During his research, he used solid copper and strong-bonded lead-core bullets and pushed them to the farthest extremes he could. “I have seen a fair number of bullets come apart, some due to bullet construction, some due to pushing the envelope on rotation,” he says. “In my experience, though, that happens way north of 300,000 RPM."

With unheard-of twist rates like 1-in-1 inches at supersonic velocity, Shafer’s been able to spin and stabilize bullets to nearly 1,000,000 RPM. And he’s found there are other benefits to increasing a bullet's RPM beyond the recommended 300,000 RPM mark.

How Rotational Velocity Affects Energy

Supersonic bullets get exponentially more energy on target the faster they go. This is why as a bullet slows in flight, it has less energy, which is measured in foot-pounds. But you can’t mess with velocity when shooting at subsonic speeds (you’re locked in at about 1,000 feet per second), and you need to get more energy from somewhere.

Shafer has found that increasing rotational velocity will give you a lot more energy in a similar way that bullet velocity does. “In traditional, supersonic rifle rounds with standard twist rates, the amount of energy that rotation gives a bullet is small, so small that we don’t bother to account for it,” he says. “For subsonics with fast twist rates, though, the energy from rotation is not only measurable; it’s significant.”

When Shafer shoots an 8.6mm subsonic bullet at 1000 feet per second, it has a kinetic energy of about 700 foot-pounds. “That’s what you’d see posted on your box of ammo,” he says. “When I twist that bullet at 1 in 8 inches, there is a 20-foot pound increase that’s so little it doesn’t matter. When I shoot that same bullet at the same speed at 1 in 3 inches, it increases to 843 foot-pounds. That’s a 17-percent increase in total energy.”

This helps bullets expand and causes way more damage. It also allows subsonic bullets to be more lethal at greater distances. But according to Shafer, “we’re not doing anything really wild until we start looking at supersonic bullets traveling out of 16-inch barrels with a 1-in-3 twist.”

That’s because the same increase in energy you get at subsonic speeds also stays true at supersonic speeds, giving a bullet 17 percent more energy than its advertised energy at more normal barrel twist rates. That means a 190-grain 8.6mm bullet at supersonic speeds goes from 2041 foot-pounds to 2,461 foot-pounds from a 1-in-3 inch barrel. That’s a 17-percent increase in kinetic energy.

At supersonic velocities, this increase in energy can increase the potential for tissue destruction and allow you to build a rifle with a shorter barrel that still maintains standard bullet performance.

So What Does This Mean For the Average Hunter and Shooter?

While most hunters aren’t concerned with researching cartridge performance, they can still take a few things away from this. One is that fast twist barrels combined with premium bullets are a good combination. It’s another reason why copper solids are probably here to stay and will continue making inroads at hunting camp.

The other thing is that short barrels don’t necessarily mean diminished performance in terms of energy. Combined with extremely fast twists, they can keep up with a longer barrel. You probably won’t see a lot of 1-in-3-twist barrels on your gun store shelf, but you still shouldn’t discount shorter barrels, especially as newer rifles and cartridges come out in years to come. However, I would be wary of companies jumping onto the short-barrel trend and subsonic cartridges with slower-twist barrels if you plan on hunting with a setup like that.

The other place rotational velocity might screw with you is when reloading. According to Shafer, there are bullet and chambering combinations out there that just don’t work. For example, extremely lightweight 22-caliber varmint bullets in fast-twist 223 Remington barrels can fall apart because of their thin jackets and cup-and-core construction. For them, you’ll actually want a slower twist. So, if you’re dreaming up a wildcat cartridge or trying to squeeze more performance out of a standard cartridge, consider things like twist rate, velocity, and bullet construction before you start messing around.

Also, handloaders should note rotational velocity will affect trajectory data when taking heavy-for-caliber bullets and slowing them down for custom subsonic loads. “When we calculate BC over distance with subsonic loads, it generally overpredicts drop at fast twist rates,” says Shafer.

What this means is that Shafer can’t use the established ballistic coefficients for subsonic shooting solutions because they’re printed for supersonic speeds. At high RPM and subsonic speeds, the ballistic coefficient actually goes up, forcing you to come up with custom data to predict bullet drop.

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