The purpose of this thread is to show the formula to calculate the required twist rate for a particular bullet length. I touched on a few other topics for explanation.

From:

www.z-hat.com/twistrate.htmYou can download a FREE program that calculates twist rate here. I had to download the zip file in order to get it to work.

Several threads have been posted regarding which twist rate to use: 1:12, 1:9, 1:8, 1:7, etc. It all depends on the bullet you are most likely to use. From what I remember, the general consensus is that a 1/9 twist barrel will suit most of the AR shooters. From the calculations below, it appears that that 1:9 twist rate will handle all bullets except the Sierra 80 grain and Hornady AMAX 75 grain.

Some barrels will stabilize bullets that are right on the calculated cusp of being stable. In this area, I can think of two things that could make or break bullet stabilization.

1. The barrel may have an actual twist rate tighter than what is labeled on the barrel. In mass produced barrels, it is not uncommon for barrel twists to be slightly off, but close enough to label it. Mass produced barrels are NOT match barrels, thus some slack may enter the picture.

2. The more the bullet's tip is yawed, or the bullet unbalanced, the less the bullet is apt to stabilize, similar to a thrown football. A football may start out wobbling and then stabilize to no yaw; this is due to more than enough spin to remove the wobbling. If the football does not have enough spin, the wobbling gets worse until the football points just about all over the place flying down the field. Bullets pretty much do the same thing, only much faster.

AR15.com Ammo Oracle has a good description of how a bullet shoots.

Basically, this is the formula for calculating twist rate: Greenhill Twist Rate formula.

Twist = 150 X D/R

Where:

D = bullet diameter in inches;

L= bullet length in inches;

R = length of bullet divided by its diameter

150 = a constant. Substitute 180 for the 150 value, for velocities exceeding 2800 fps, which is most AR loads.

Below are calculations using some of the more popular bullets, and the twist rate required to stabilize each. The program rounds off the calculations to the nearest whole number. I calculated using a calculator using the "180" constant.

The lower the number, the faster the twist rate.

9 is faster than 12.

9 is faster than 9.17.

12 is faster than 12.12.

M193, 55 grain, bullet length = 0.745”

Calculated twist rate = 1:12

When done by hand, twist rate = 1:12.12

M855, IMI, 63 grain, bullet length = 0.872”

Calculated twist rate = 1:10

When done by hand, twist rate = 1:10.36

SS109, 62 grain, bullet length = 0.866”

Calculated twist rate = 1:10

When done by hand, twist rate = 1:10.43

Hornady match, 68 grain, bullet length = 0.985”

Calculated twist rate = 1:9

When done by hand, twist rate = 1:9.17

Sierra match, 69 grain, bullet length = 0.898”

Calculated twist rate = 1:10

When done by hand, twist rate = 1:10.06

Hornady AMAX, 75 grain, bullet length = 1.070”

Calculated twist rate = 1:8

When done by hand, twist rate = 1:8.44

Sierra match, 77 grain, bullet length = 0.985”

Calculated twist rate = 1:9

When done by hand, twist rate = 1:9.17

Nosler HPBT, 77 grain, bullet length = 0.990”

Calculated twist rate = 1:9

When done by hand, twist rate = 1:9.12

Sierra match, 80 grain, bullet length = 1.090”

Calculated twist rate = 1:8

When done by hand, twist rate = 1:8.29