Why Do Guns Fire Spinning Bullets and What Is Angular Momentum?

A spinning bullet flies farther and truer than it would without the spin. And if your favorite sport is football rather than shooting, just about everything I’m going to say about spinning bullets also goes for spiraling passes.

The fact that a spinning bullet or football goes farther may sound strange, because you’d think that the range would depend only on the amount of energy it gets from the gunpowder charge or the quarterback’s arm. But bullets and footballs have to fly through the air, and air drag plays an important part in any projectile’s trajectory, whether it is fired from a handgun, rifle, machine gun, howitzer or arm.

First, let’s see how a gun makes the bullet spin.

Running the length of the inside of the barrel are spiraling grooves, called rifling. As the bullet passes through the barrel, these grooves cut into it, making it rotate to conform to the spiral. Some guns have grooves that twist to the right and some have grooves that twist to the left; it doesn’t matter.

And no, they don’t twist one way in the northern hemisphere and the other way in the southern hemisphere.

Early bullets were round balls of lead, like miniature cannonballs. Bullet-shaped (Techspeak: cylindroconoidal) bullets were developed around 1825, when it was found that they maintained their speed better in flight. That’s because for a given weight of lead an elongated, tapered-nose shape meets with less air resistance than a round ball; it’s streamlined.

But there’s a problem with elongated bullets that spherical bullets don’t have. When an elongated bullet is fired, any tiny irregularities on its surface can catch the air and push it slightly sideways, so that its nose is no longer pointing straight ahead. This slight misalignment increases the air resistance on the forward side, which turns the bullet even more. Pretty soon it is tumbling end-over-end, which causes even more air drag, seriously shortening its range and pushing it off-course. Thus, both distance and accuracy suffer.

That’s where the rifling comes in. If the bullet is spinning properly around its long axis as it flies, it resists any change in its orientation or direction of flight. The reason for that is that a heavy, spinning object has a lot of momentum. Not only does it have momentum along its direction of travel (linear momentum), but because of its spin it also has rotational momentum, or what physicists call angular momentum. And momentum, whether linear or angular, is hard to upset.

In fact, the momentum of an object will remain unchanged unless and until it is disturbed by some outside force. (Techspeak: Momentum is conserved.) The spinning bullet, therefore, will maintain its angular momentum by spinning with its axis in the same direction for as long as it is in the air, because there is no outside force to disturb it. Those tiny surface irregularities are now peanuts compared with the bullet’s substantial amount of angular momentum.

With its nose pointed straight ahead, the projectile encounters less air resistance and thus flies farther and truer. When it ultimately hits an object, its momentum, both linear and angular, still won’t disappear, but will be transferred to the unfortunate target, or in the case of a football, the fortunate receiver.

International law actually requires that bullets spin. Otherwise, a tumbling bullet might hit its victim sideways, doing more damage than if it had made a nice, clean, round hole. It’s just one of those niceties of war: If you’re going to kill somebody, please do it neatly.

The Geneva Convention spells out certain other niceties about how to kill people. For example, because lead is soft and deformable, it can go splat when it hits its target, again producing a very unsightly hole. So bullets have to be jacketed with a harder metal, such as copper.

The world’s military establishments gladly comply with that requirement, but it’s not because of any humanitarian motives. It’s because modern military assault weapons fire their bullets at such high speeds that if they weren’t jacketed with high-melting copper the lead would melt from friction with the air, making them fly erratically and miss their targets.

After all, a clean, round hole in an enemy is so much preferable to no hole at all.