Bore Rifling

Rifling refers to the spiral grooves that are cut into the internal surface of a gun barrel. Rifling helps impart a spinning motion to a bullet when it’s fired. 
A spinning bullet is much more stable in its trajectory, and is therefore more accurate than a bullet that doesn’t spin. This is exactly why is is better to throw a football in a spiral. How does the spin make the flight of a bullet more stable? By Angular Momentum or gyroscopic inertia.

​Angular momentum is the amount of rotation in a body and characterizes an objects resistance to change in rotation. Similar to how a body moving laterally has linear momentum, a spinning body has angular momentum. The quantity of rotation of a body is the product of its moment of inertia and angular velocity. 
Angular momentum is nearly conserved.
  • rotational inertia of mass at distance r: I = mr²
  • angular momentum: L = I ω

Is there an ideal amount of spin?

There is in fact an ideal amount of spin to put on a bullet. Too little spin will cause the bullet to not fly straight. But more interesting effects happen if there is too much spin. 
  • Over stabilization will cause bullet drift and affect accuracy.
  • Excessive spin can cause the pullet to destroy itself due to centrifugal forces.
​The ideal twist rate of the bore is given by the Green Hill Formula, which factors in bullet diameter, length, specific gravity, and a constant C  (C=150 for velocities up to 2,800 fps. 180 for velocities over 2,800 fps).
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