BulletTwist

Calculate what barrel twist is needed for a stable projectile or decide what bullet is needed to shoot your bullet with stability in your existing twist. This uses a formula that is more accurate than Greenhill, and also considers atmospheric conditions. You may either set the current conditions, or use the default worst-case ones to ensure stability for universal use.

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Also see my other apps:
BulletFlight M
BulletFlight L1
BulletFlight L2
GunSounds
HandgunSounds
MILDOT 10