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.
taking note of this, it is much easier to under stablize a bullet then it is to over stabilize a bullet. some people actaully take adjantage of this, the belief being that under stabilized (but still accurate) bullets will tumble through game causing extra damage. examples of this can be seen in some millitary rifles using extra heavy bullets in slower rate of twist barrels and in some hunters using big 60 grain + bullets in .223 diameter centerfires with standard barrel when used against larger game. its also believed that bullets expansion is emphasize at high rpm's, produced by fast rate of twist barrels.
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So as a person that doesn't reload, does this mean that certain barrels are optimized for certain weight bullets? Is there a rule of thumb for determining what off the shelf load would shoot best? As a hunter and not a target shooter, is this something I really need to be concerned with? I assume there is no danger in over or under stabilizing - correct?
Last edited by clayshooter25; 09-16-2010 at 06:59 PM.
So as a person that doesn't reload, does this mean that certain barrels are optimized for certain weight bullets? Is there a rule of thumb for determining what off the shelf load would shoot best? As a hunter and not a target shooter, is this something I really need to be concerned with? I assume there is no danger in over or unxer stabilizing - correct?
With most factory guns, no concern. For example. you rarely see a 308win with a 1 in 10 barrel, and rarely see anyone shoot more than 180gr load. No big deal.
It could get harry when buying a target AR-15. Some come with 1 in 7 or even 1 in 6 barrel. Made for shooting 70-90gr .223 bullets. Where most factory ARs are 1 in 9 made to shoot 50-65gr bullets.
ita not weight of a bullet that determines what twist ya need its the length (though in typical cases a longer bullet is heavier). and velocity also plays a part in what twist ya need.
example: a factory 7 RUM with a 9 twist will stabilize a 200 gr wildcat nicely, however a 7mm RM will not because the rums added velocity assists in stabilization.
just a a 22-250 will stabilize a heavier bullet with a 9 twist than a 223 will.
RR
If the bullets jacket will stand the rpm's of a given caliber and twist, its hard to overstabilize a bullet. an added bonus to a fast twist is better terminal performance because bullet spin also helps expansion. the only downside is just a tad less velocity with the same loads.
I have 223's with 8,9,10, and 12 twists. I shoot mainly ultra-max reman 55 gr sp's in all those rifles, the 8 twist is a bit slower, even though it has a 24" barrel, but accuracy is better as is terminal performance (it puts a bigger exit hole through a groundhog.
anytime I build a rifle I'll always opt for a faster twist than I think I have to have if its available.
RR
Rifle Barrel Twist Rate?
