Question about twist rate and stuff (math warning)


February 20, 2010, 12:57 AM
The first being that, when using the Greenhill formula to calcuate the optimum twist rate for a bullet, you divide 150 by the bullets (length/width) and then multiply by the width or 150 times the width squared, all divided by the bullet length.

Now, that will tell you the optimum twist rate for a bullet of that particular width and length but if you already know the twist rate and bullet width and want to find the optimum length bullet for that twist rate you just do 150*width squared divided by twist rate and it will tell you the length you need.

It's pretty hard to shop for a bullet based on it's length though since bullets are measured in width and weight. I know that using that formula, the optimum twist rate for a .308 168gr Sierra match king (which is approx 1.2 in long) is 11.858 inches. Now if I take 168/1.2 I get approx 140 grains of weigh per inch. So if I use my reverse formula and find out that the optimum bullet length for a barrel of 1 in 10" (which mine is) is 1.42296 inches and multiply that by the 140 grains per inch idea I get approx 199 grain bullet.

Am I right here? Is that a good way to judge what WEIGHT bullet is right for a barrel. What is more important to twist rate, bullet length or bullet weight? What I mean by that last question is, is the bullet stabilization more affected by an increase in bullet length or bullet weight?


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Zak Smith
February 20, 2010, 01:05 AM
I recommend going by the bullet manufacturer's recommendation. The formula will only in theory be optical for one type of bullet construction, which is not necessarily what you are using.

For example, the 155 Lapua Scenar is longer than a 175 SMK.

Stabilization has to do with the amount of rotational momentum vs. the length because the former resists the transverse torque about the bullet base of the latter.

The shorter the bullet, the less transverse torque trying to upset the bullet.

The more mass distributed further from the axis, the more rotational momentum it will have given the same RPM.

Bart B.
February 20, 2010, 01:23 AM
Typically, a given bullet spun at the best RPM rate to keep it stabilized is best. Whatever muzzle velocity and twist it takes to stabilize the bullet will work.

Folks have shot 150-gr. spitzer boattail bullets out of 30 caliber 1:13 twist barrels at muzzle velocities at about 3000 fps. Great accuracy is at hand doing so. But that same bullet needs only 2750 fps to spin at the same rate from a 1:12 twist barrel. Such bullets need to spin about 165,000 RPM to stabilze them at most muzzle velocities. A bit faster RPM's when they leave over 3500 fps.

RPM's are calculated by:

Muzzle Velocity times 720 divided by Twist in inches.

Heavier and longer bullets need to be spun faster; lighter and shorter ones, slower.

That Greenhil formula was originally meant for bullets leaving under 2000 fps. With todays higher muzzle velocities, it's not all that good of a tool.

Bullet makers know what twist and muzzle vleocity is needed to stabilize them. Sierra Bullets probably has the best information.

February 20, 2010, 12:49 PM
Ok. Thanks for the info guys

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