Why Does one Impart 'spin' to a Bullet?

A rifle Barrel has a spiral groove which imparts spin to the bullet. Why?
- Ajay Gautam (age 18)
Sonipat , Haryana , India

Hello Ajay,

Because it improves the stability of the bullet's trajectory.  

All objects have three principal axes of rotation with corresponding moments of inertia.   Call them I₁ ,  I_2,  and  I₃ with I₁  <  I₂  <  I₃ .   It turns out that if an object is spinning about I₁  or  I₃  then its angular motion is stable against small perturbations.  On the other hand if it is spinning about I₂ then it will 'tumble' chaotically.   For a bullet, tumbling is not a good thing since the varying wind resistance will tend to move it off course.   For the special case where I₁  =  I₂  the object will be stable when spun along the common  axis.

This phenomenon is described in 

 

LeeH

(published on 01/05/2015)