Originally Posted by
clayshooter25
I understand what the Twist Rate number is telling me, but can someone explain what I use this number for? What does it help me to determine?
Thanks,
Michael
You can over stabilize a bullet or under. Under can cause bullets to loose spin and keyhole. Or just bad accuracy.
Heavier bullets require faster spin, lighter bullets require less. But Specific gravity, type of material, velocity plays the role in the best stabilized twist.
A shooters rule of thumb is based off the greenhill formula.
The
Greenhill Formula,
Twist=CD^2/L X SqrRoot(SG/10.9)
where:
- C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
- D = bullet's diameter in inches
- L = bullet's length in inches
- SG = bullet'sspecific gravity(10.9 for lead-core bullets, which cancels out the second half of the equation)
Let say you have a 300RUM, generally has 1 in 10 twist, optimizied for 180-200gr bullets. Where a 308 generally has 1 in 12 made more for a 150-165gr bullet.