Faster Than a Speeding Bullet

By Eric M. Jones

MY FRIEND HAD SUFFERED FROM chronic back pain for years. He’d gone to many doctors and chiropractors, had physical therapy, had a lift put into one shoe, did exercises, and the pain just never improved without a perpetual diet of painkillers. Doctors were starting not to believe his continual complaints about his back pain.

Finally, one doctor reluctantly agreed to x-ray his lower back. A few minutes later the doctor appeared with the film in hand, pointed to a white spot on the image and said to my friend, “So when were you shot?”

He claimed he didn’t remember getting shot. Some “negligent celebratory gunfire” .22-caliber stray bullet had been flying around and found his lower back. Bullets cause damage long after they are fired—

Dutch owned a Cessna 195 with a big radial Jacobs engine that he’d bought in the mid 1960s. It had paint the color of orange sherbet trimmed in vanilla ice cream. Dutch and Leo loved to fly into Mexico, to explore the backcountry and sample the local food—and if truth were to be told—visit what they call in Mexico, “full-service towns.” They knew many secret places in Mexico that one could only reach by horse, burro, foot or airplane.

While camping in the desert, Leo came across an iron cannonball, probably from the French-Mexican war of 1861–1867. He said it looked exactly like a blackened cantaloupe—and Leo decided to take it home.

The cannonball was stuffed firmly among the baggage in the rear of the airplane, all the camping gear was stowed and the airplane preflighted.

Dutch brought the big Jacobs engine to life, warmed up the oil and did his flight control, instrument and engine checks. In a few minutes they were bumping down the dirt runway gaining speed. The runway became even rougher. But Dutch couldn’t raise the tail. A little more speed and the tail still seemed glued to the ground. Something was wrong. The end of the runway was coming up much too fast and Dutch knew he had to make a decision—fast.

Dutch decided the 195 simply wasn’t going to fly so he chopped the throttle and stood on the brakes. This brought the tail up, put the propeller and nose into the dirt, and the cannonball came roaring from some secret hiding place in the tail— missing Leo’s head by a finger’s breadth—and obliterated the instrument panel.

It was almost a year before the big Cessna got back to the United States and it was a long time from touching off that French cannon to shooting down an aircraft.

***

Many people have a fundamentally incorrect notion of how bullets work. This leads to some strange errors in decision making. “So what happens to the bullet when you shoot it up into the air?” police officers often ask young revelers. They usually reply, “I thought the bullets just disappeared.” You can bet those guys firing AK-47s into the air think the same thing.

Well, they most definitely don’t disappear. But bullets smaller than .50-caliber (or so), fired straight up won’t kill you, even if they smack you directly on the head when returning to Earth. If they hit you, the flat tail of the bullet or the side but not the point will likely strike you.

Tests on this subject have been done for many years. The reason this happens is that a bullet fired approximately straight up leaves the gun barrel with enormous velocity, perhaps 350m/s for a pistol. But returns to Earth only at its terminal velocity, perhaps 50m/s. Because the kinetic energy is proportional to the velocity squared, the bullet comes down with 2 percent of the energy it had when it left the barrel. Usually, this is not lethal if the bullet is fired vertically. (Bullets fired at lower angles are much more lethal because they retain more of their kinetic energy—proportional to the cosine of the firing angle).

So why would a bullet that goes up pointy-end first come down flat-end first? Because of the enormous rotational speed that gyroscopically stabilizes the bullet. Most people ignore this aspect of bullets fired from rifled gun barrels, because it is extraordinarily hard to observe. The bullet leaving a 9 mm pistol is spinning at nearly 100,000 rpm. Assault rifles typically spit deadly bullets that spin at one quarter-million rpm. This adds to the destructiveness of bullets hitting targets, as the bullets will fragment from centrifugal force the instant their shape is flattened upon impact. Bullets probably don’t tumble at all.

For most of their history, bullets have been round lead balls. These bullets were quite loose in the barrel, and they used a cloth patch to center and retain them in the rifle bore. Accuracy was poor at distances over ~50 meters, and were frequently extremely wild shots if the side of the ball touched the wall of the gun barrel. If so, the ball could spin ferociously on any axis and curve off at any angle when it left the muzzle. This required a calm precision in loading, which was more difficult in the heat of battle.

Historically—shortly before the American Civil War, gun barrels were grooved
internally to rotate the bullet and thus gyroscopically stabilize it as it
travelled out the gun barrel and the bullets were reshaped to seat in these
rifling grooves, like modern bullet forms. This was an enormous improvement in accuracy.

Battleship main gun projectiles leave the barrel at spins over 5,000 rpm. As illustrated below, one thinks the nose hits the target. But actually at higher angles of elevation, the projectile can be in any orientation when it hits. This has caused malfunctions with some projectile-fuse designs, especially when the impact area is soft. The calculation of battleship gun trajectories was a chief driver of the early computer revolution due to a dozen or more variables affecting the accuracy of the shot.

So how do they get the twisted rifling grooves into the barrel anyway? And for that matter how the hell do they drill the hole in the barrel? If you drilled the hole by rotating the drill bit, the hole would just wander off into the side and the hole would be unusable. But if you rotate the barrel and rigidly clamp the drill, the hole will be straight. This is counterintuitive because it would seem that either rotating the barrel or rotating the drill bit would do exactly the same thing, but it just isn’t so.

The grooves that cause the projectile to spin are usually machined by pushing a guided broach down the barrel. Sometimes the rifling twist, or pitch, increases from breech to muzzle. This causes the projectile to gradually spin faster as it moves out of the barrel.

Small-gun manufacturers and experts don’t think this variable pitch is particularly worth the effort, especially as the tooling complications are far greater. (Factoid: The 6.5 mm Carcano Model 91/38 that was used to assassinate John F. Kennedy had variable pitch rifling.) In big battleship guns, however, this variable pitch rifling controls how the projectile acceleration and spinning-up (its second moment of inertia) are coordinated, reducing the required strength and weight of the gun barrel. This provides real advantages for big guns, and was a hush-hush top secret throughout the twentieth century.

There are other methods of adding the rifling. Beretta and many other quality small-arms makers bore their barrels oversized and then hammer-swage the barrel onto a mandrel that contains the rifling and reflects the final bore size. Then the mandrel is pulled out. This technique, of course, makes variable-pitch rifling impossible.

But spinning the bullet doesn’t add kinetic energy to the round, and sometimes kinetic energy is the only tool you need. Most battle tanks now have smooth bore cannons, mainly to fire winged projectiles and “sabot” rounds. Sabot rounds are relatively lightweight kinetic penetrating tungsten or depleted uranium arrows that can accelerate to tremendous velocities because they weigh so little compared to big heavy projectiles. They are centered in the breech and barrel by parts that fall off when they leave the muzzle of the cannon. Whereas the standard rifled-bore cannon rounds have a muzzle velocity of 250m/s, the sabot rounds have a muzzle velocity of nearly 2000m/s.

So why isn’t the rear of the bullet more streamlined? In fact, why isn’t the projectile shaped like a teardrop for maximum aerodynamic streamlining? Well, the streamlining of a bullet is usually designed for supersonic speeds. At subsonic speeds it would make sense to have a pointy tail or a teardrop shape. But I also suspect it is because of a persistent illusion that a streamlined rear will allow gas to escape as it is propelled out the barrel. The explosion, it is believed, needs something flat to “push against.” This is a deeply held but mistaken notion found often in mechanical engineering practice.

A note on mortars: They have better streamlined forms; have fins on the tail to stabilize their flight and guarantee they hit head first; work at any angle; and launch from thin-walled lightweight portable smoothbore launchers. In short—they do everything right.

A note on bullet spins: Some people claim that a bullet won’t maintain its spin long enough to return to Earth spinning. While I agree that there has been virtually no unclassified science on the matter, gyroscopes are gyroscopes. There are several amazing You Tube videos on the matter, where bullets are fired into lake ice and spin a remarkably long time. 

Eric M. Jones is the Contributing Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur. His Internet business PerihelionDesign, builds and sells products, parts and materials to the home-built experimental aircraft community.