For the most part, this is a complicated, and poorly understood subject.... It has always fascinated me, and the lack of good information on subsonic ballistics has been a constant source of frustration to me.... A while ago, I discovered a Twist Calculator in the Border Barrels (Scotland) website which works not only for supersonic flight, but also for subsonic and the transition in between.... It has numerous inputs, and I would suggest you check it out.... http://www.border-barrels.com/barrel_twist.htm

First of all, let me state that I am NOT an expert in this field.... I have, however, done a lot of reading on the subject, to try and get a grasp of what is going on.... The original "standard" method of calculating the optimum rifling twist was the Greenhill Formula which dates back to 1879.... There have been many derived since, and one of the better ones is the Miller Rule, published in 2005.... However, even with that modern formula (probably because he had no interest in it), all velocities below the speed of sound used a velocity of 1120 fps.... The Border Barrels calculator, however, presents the optimum twist in graphic form from 500 fps up, and according to their website:
This program is basically a slick version of Robert McCoy's "McGyro" DOS program, written in the late 1980's when he worked for the U.S. Army Ballistic Research Laboratory at the Aberdeen Proving Grounds. It was later improved by William Davis Jr. of Tioga Engineering and the claim was an accuracy of 5% for super and subsonic velocities, and 10% for trans-sonic velocities.
The basic output of the calculator is a graph for a Stability Factor of 1.5, but you can also input the twist you are interested in, and it then presents a second graph giving the Stability Factor for that twist.... A Stability Factor less than 1.0 is unstable, and greater than 4.0 is considered "overstabilized" which simply means that any imperfection (ie imbalance) in the bullet can cause "precession" or wobbling, which tends to open up the groups.... The military chooses Stability Factors of 1.5 to 2.5, and benchrest shooters often opt for only 1.3, but 1.5 is generally considered a good design point....

Let's ignore velocity and bullet shape for a moment.... There are two basic concepts you have to understand about rifling twist....

First, you have to realize that all the twist equations originally worked in "calibers".... The length of the bullet, and the resulting optimum twist, were relative to the diameter of the bullet.... For a given shape of bullet (ie length to diameter ratio), the required twist was the same IN CALIBERS.... If a 15" twist was the best in a .25 cal (with a .50" long bullet), then you need a 30" twist for a .50 cal (with a 1.00" long bullet)....

Second, if you increase the length of the bullet (in a given caliber), you need to spin it faster in order for it to be stable.... Imagine how fast you would have to spin a pencil to make it fly straight (without vanes or feathers) and you get the idea....

Once you understand those two ideas, you are well on your way to understanding twist.... In the following posts, I am going to give some examples, using the Border Barrels Twist Calculator to produce the graphs....

Bob