Why must rockets and bullets spin

[alchemist]

why must rockets and bullets spin when they are in motion in order to move in the intended motion?
what does the spinning motion do?
does it produce a force to matian the centre of gravity at the centre? and what if the bullet is a perfect sphere, does it still need to spin?

 

[Michael D. Sewell]

what if the bullet is a perfect sphere, does it still need to spin?

It is not too uncommon for a modern rifle of high quality to shoot a five-shot group of less than 1 inch at 100 yds.

Without spinning, the long pointed bullets that we use today would tumble in flight. Each shot would impact in a different county(slight exaggeration).

If a spherical projectile is fired from a rifled bore with the proper "twist", one can expect to get a five-shot group at 100 yds of 2" or less with a rifle of decent quality. Rifles shooting round balls were capable of this kind of accuracy over 100 years ago.

If the same type spherical ball is fired from a smooth bore rifle, the group sizes run from 1 to 3 FEET at 100 yds. Early rifles weren't much better than long bows in battle, and in many situations, weren't as good.

-Mike