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Why does a bullet spin?

  1. Jul 30, 2010 #1
    I was wondering why a bullet spins. From what I read a spinning bullet shoots straighter, but I don't really understand why since the Magnus Effect would cause it to drift or does the gyroscopic inertia compensate for this?

    Also, does gyroscopic inertia cause the rotational axis to be stable at a position in space or does the effect only maintain that the direction of the rotational axis doesn't change but can be shifted?

    Thanks.
     
  2. jcsd
  3. Jul 30, 2010 #2
    A spinning axis gives stability along the axis (gyroscpic inertia). It's a directionnal stability only. Regular inertia counters shifting.

    But in which direction would the Magnus effect steer the bullet? Answer : neither because the spinning axis is along the trajectory (unlike to a baseball). There is no shifting except gravitationnal and wind.
     
  4. Jul 30, 2010 #3

    Andrew Mason

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    A bullet spins because the gun barrel is designed to put spin on it. It has spiral grooves that dig into the surface of the bullet (lead or copper usually) and cause the bullet to rotate as it goes down the barrel.

    A rotating bullet is more stable than a non-rotating one because it has angular momentum. In order to change angular momentum, a torque has to be applied to the bullet (torque = time rate of change of momentum so if no torque, angular momentum does not change).

    AM
     
  5. Jul 31, 2010 #4
    Here:

    http://en.wikipedia.org/wiki/Magnus_effect

    They explain that due to cross winds the bullet will yawn and its nose (and axis of rotation) will yaw and end up pointing in a different direction than its direction of motion (so Magnus effect should occur?) so would the added gyroscopic inertia more than compensate for this effect?
     
  6. Aug 2, 2010 #5
    In the early days of musketry, the bullets were spherical and ejected from the barrel with random spin. The Magnus effect causes such balls to careen (curve) unpredictably. It was soon learned that by causing the spin to be along the velocity vector (by "rifling" the barrel), Magnus effects are reduced and accuracy is greatly improved. Later still bullets were "streamlined" or altered from spherical shapes, and the spin also keeps the noses of such streamlined slugs facing into the wind from barrel to point of impact.
     
  7. Aug 2, 2010 #6
    In that diagram, the ball is rolling end over end, like a baseball. Bullets spin around a horizontal axis, not a vertical axis.
     
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