# Stability of a bullet in vacuum

Imagine you fire a bullet in vacuum (in the presence of gravity). Would spinning bullets (riffled barrel) be more stable than non-spinning bullets?

In other words, I would like to know if the aerodynamic force exerted in bullets by air, is the sole reason of having riffle in barrels?

• Can we get a definition of stability first? Are you talking about stability of the axis of rotation? If so, I should think that in air, lack of stability has real effects that do not occur in a vacuum -- e.g. a tumbling bullet in air will feel much more drag than a stably spinning one. Dec 29, 2021 at 15:33
• @Ross Presser Yes, stability of the axis of rotation, but I am asking about the cause of the instability, not its effect.
– Ebi
Dec 29, 2021 at 16:33

"aerodynamic force(s) exerted in bullets by the air, is the reason of having riffle in barrels?" Short answer: yes.

Longer answer/explanation: in air, having the bullet rotate (due to rifling) causes the conical front to stay aligned (mostly/approximately) with the direction of motion, improving the aerodynamic characteristics: reducing drag leading to increased range; increased accuracy.

In a vacuum these aerodynamic considerations are gone so it should not matter whether the bullet is rotating or not.

A related thought experiment: imagine two asteroids traveling through space, one rotating with axis of rotation aligned with trajectory, the other not rotating. I can't think of any reason why one is more stable than the other (to first order).

• isnt it gas produced by the bullet's own charge that makes it spin (as opposed to ambient air in the barrel)? if so, it would spin anyway.. Dec 29, 2021 at 17:26
• @tobalt No, it is not the gas causing the spin, it is the rifling inside the barrel which are deliberately manufactured to spin the bullets (helical grooves inside the barrel). So, no riffling, no spin.
– Ebi
Dec 29, 2021 at 17:29
• Re, "...should not matter whether the bullet is rotating or not." Except, the shapes of many real bullets and other kinetic projectiles is not only dictated by how they interact with the air, but also on how they interact with the target. Armor piercing rounds and expanding rounds and probably any kind of fuzed, explosive projectile won't perform as intended if they don't hit or approach the target in the right orientation. Dec 29, 2021 at 18:01
• @solomon you cut out the key first part, "In a vacuum these aerodynamic considerations". The bullet's orientation will not change in a vacuum, therefore all these other considerations you mention would not be affected Dec 29, 2021 at 18:05