General Question

Harp's avatar

Why does rifling improve the accuracy of a spherical bullet?

Asked by Harp (19038 points ) August 9th, 2009

Rifling (the spiral grooves on the inside of a gun barrel that impart a spin to the bullet) was invented back when bullets were spherical, but they increased the accuracy range of guns by a factor of ten. I can understand the advantage of spin in the case of an elongated bullet to keep it from tumbling, but how would it matter in the case of a smooth sphere?

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20 Answers

thrice2k3's avatar

As I recall and I could be wrong but the spin allowed for a more predictable flight path… in kinda the same way a pitch is manipulated… or not… by a pitcher in baseball. Dictating the spin sorta dictates the eventual flight path…

ragingloli's avatar

have you ever kicked a curve football? the way the ball spins determines its flight path, as the movement of the surface influence s the drag. if the rotation axis is horizontal, one side of the ball/bullet will move against the air, the other side with the air. the side moving against the air will experience overpressure, the side moving with the air will experience reduced pressure. both combined will cause the ball to move towards the lower pressure.
the rifling most likely gives it a longitudinal rotation axis which prevents the bullet from moving left/right or up/down.

PerryDolia's avatar

I don’t know the answer either, but I bet it has something to do with the fact that the bullets were only approximately spherical.

This means, without rifling, they would have the tendency to wobble around their center of mass (not geometric center) during flight. This would make them curve.

Rifling would cause them to rotate in a specific way during flight, reducing the wobble. The rifling essentially de-sphericalized the bullet and made it more oblong and caused it to rotate, thus making more accurate.

jaketheripper's avatar

rifling creates a consistent spin that doesn’t affect the trajectory. If the bullet spins in any direction other than on the axis of its flightpath it will create a significant curve in its path.

Harp's avatar

In the case of baseballs and soccer balls, it’s the stitching and grooves that create the pressure differentials that affect trajectory, as I understand it. If the bullet were an ideal sphere, would there be no improvement in accuracy?

ragingloli's avatar

i made a little sketch to illustrate.

Jayne's avatar

@ragingloli; longitudinal rotation does not prevent motion to the sides, it just does not create lateral motion like other rotations do. However, (I think) like @jaketheripper I would hypothesize that the longitudinal rotation at least makes it more difficult, due to gyroscopic inertia, for the bullet to acquire those lateral rotations due to striking the side of the barrel etc.

filmfann's avatar

If it was a smooth sphere, it wouldn’t have much effect. But bullets are not smooth spheres.
The hardest baseball pitch to hit, or throw, is the knuckleball. The idea of a knuckleball is that the ball doesn’t spin at all. The result is it bobs and weaves with frighteningly irregularity. Catchers hate them, because they are a bitch to catch.

ragingloli's avatar

that is the point, isn’t it? to create longitudinal rotation so the sphere does not pick up any other rotation, which would, due to aerodynamic effects causing different pressures on opposing sides, cause the sphere to fly a curve.
If the sphere was smooth, it still would have the same effect as it would still create friction with the air.

Harp's avatar

@filmfann The knuckleball, again from what I understand, can’t be devoid of spin; it has to rotate, but less than one full rotation in its trajectory. It’s the slow rotation of the stitching that makes the trajectory crazy.

LuckyGuy's avatar

I agree with @ragingloli here. Even if the ball was perfectly smooth you still have to contend with the boundary layer. If the ball is spinning longitudinally the top and bottom have the same pressure acting upon it. You also have the moment of inertial to keep that balance. Yourr accuracy shot to shot would be improved.

The Marui Air Soft BB guns use 6mm plastic BBs that are very smooth. Marui came up with a design called Hop Up that allows the BB to travel farther by putting a top spin on it. The BB flies out straight and then suddenly hops up giving further distance. Both standard and Hop Up have the same muzzle velocity, 90m/s, But the Hop Up go farther. Sometimes the Hop Up is off a little and the BB curves one way or the other. That is proof that a spinning ball can be influence its flight.
If you spin it longitudinally every time, the accuracy and repeatability would increase.

Here is a discussion on Boundary Layer

ragingloli's avatar

and the inevitable uneven weight distribution causing it to wobble due to centrifugal forces.

Harp's avatar

If there were an uneven weight distribution along the “equator” of the spinning ball, though, wouldn’t the spin create wobble rather than prevent it, because of the centrifugal forces?

ragingloli's avatar

yes, but it still would be more accurate than lateral spin.
longitudinal spin does not prevent wobble. it prevents curved flight paths.

LuckyGuy's avatar

If it is spinning you would have slight wobble around the equator. If not spinning you can have a sustained side-load throughout its flight path which reduces accuracy.

DarkScribe's avatar

Who on earth told you that rifling in a musket increased accuracy by a factor of ten? That is far fetched. Rifling dates back to the fourteen hundreds and it gave a moderate improvement in accuracy, nowhere near as much as you seem to believe. I have shot (and still own) both rifled and smooth bore black powder muskets and the difference is definitely moderate. If you want to understand why a spinning projectile is more accurate, it is basically the same reason that a top or a gyro won’t fall over when rotating at speed. It keeps the same attitude.

Shuttle128's avatar

@DarkScribe Exactly right. The rotational inertia of a rotating object tends to keep an object headed in the same direction. Any tendency to change direction is opposed by a gyroscopic torque.

@worriedguy The effect of a rotating cylinder in air does cause lift due to boundary layer adhesion to the surface. @ragingloli is correct though. The changes in pressure on a sphere is due to the differences in apparent velocities of the air surrounding the sphere. In the case of lateral rotation there is no difference in velocities on any side of the sphere. Besides a heavy musket ball travelling at such high speeds the speed of rotation is much less than the velocity of the ball. The aerodynamic forces on the ball due to rotation would be much smaller than those caused by separation drag and the rotational inertia. The reason hop up style rotation is so unstable is because it is not in line with the direction of motion. I know exactly the instability you’re talking about. I had an MP-5 TM that would begin to loose accuracy at about 30 ft due to the instability of the hop up.

I do wonder though… seems as if a longitudinally spinning ball may have some drag advantages over a musket ball fired through an unrifled barrel. The vortex created by the spinning ball might actually decrease the separation drag of the ball.

Jayne's avatar

@Shuttle128; the rotation of an object does not keep it moving in the same direction; it has absolutely no effect on the motion of the objects center of mass (in a vacuum). What it does do, as with a gyroscope, is oppose the rotation of the object about a different axis, which in this case means preventing it from rotating it about an axis which, due to interaction with the surrounding air, may alter its course.

Shuttle128's avatar

@Jayne The rotational inertia of an object orients itself along the axis of rotation. Any disturbance in orientation causes a torque that opposes the disturbance. It is a form of directional stability. You are correct a force imposed on a spinning object can change the direction of motion of the center of mass but the orientation of the object will remain unchanged. The directional stability of an object traveling through air is rather important. Instabilities cause the flight of a projectile to be unpredictable. What I wrote in my first post wasn’t the clearest explanation, my apologies.

Webster's avatar

A ball (sphere), by the very virtue of its shape, is inherently stable. It cannot turn sideways (no sides), nor can it topple end over end for the same reason.

If one could shoot a perfectly spherical ball without any spin whatsover, it would fly as true as a ball shot from a rifled barrel. An example of this, is a well hit golf ball from a #1 wood.

In reality, this is not possible, and balls fired from a smooth bore will, because of ball imperfection, unequal friction between the ball and the barrel, etc., have some uncontrolled spin.

The result is the ball veering from the sight path, similar to a hook or slice in golf, or a curve ball in baseball.

This is caused by what is defined as the “Magnus Effect” (the lateral force exerted on a round spinning object moving through a liquid or gas)

To prevent the Magnus Effect, one of two options are available;

1) Fire the ball with no spin (impossible from a practical perspective)

2) Spin the ball so that the axis of spin is aligned with the barrel axis/flight path, thus nullifying any Magnus Effect.

Unlike spin imparted on a bullet, it doesn’t really matter how fast the ball is spun, as long as the axis of spin aligns with its barrel axia/flight path. One turn in 66” is adequate.

Shooting patched balls from rifles with a shorter twist will work fine, but it may be necessary to reduce the charge so as to not have the patch strip in the rifling, resulting in dimished accuracy.

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