What this program does
The program will calculate what barrel twist is required for a Stability Factor S = 1.5 and plot a graph of this twist as a function of muzzle velocity. If a barrel twist is entered, the program will also plot a graph of Stability Factor as a function of muzzle velocity for this rate of twist.

Using the program
Try to fill in as many of the bullet dimensions as possible, even if the value is zero, as in the boat-tail length for flat based bullets. If you miss out some dimensions, the program will try to make intelligent decisions about what those dimensions are, but it is best to be clear about the bullet dimensions in the first place.

Effort by the program to be helpful and flexible can mean that too much information will lead to conflicts. They are resolved in this way. Any entered value of base diameter will take precedence over that calculated from a boat-tail angle and boat-tail length. If no value for the nose length is entered, a flat nose is assumed. Values entered for the local atmospheric pressure (anything other than 29.92 in. Hg) will take precedence over local pressures estimated from the barometric pressure and the altitude.

The outputs of the program are graphs of required barrel twist for a Stability Factor S = 1.5 -vs- muzzle velocity and the Stability Factor produced by a barrel of a given twist -vs- muzzle velocity. Do NOT assume from these graphs that they show the twist required to keep the bullet stable as its velocity reduces down range, or (for the second graph) the stability factor of the bullet as its velocity reduces down range. The rotational velocity of the bullet diminishes at a much slower rate than its translational velocity, so the stability of the bullet (usually) increases as it moves down the range. The graph labels say "Muzzle Velocity" and that is what they mean.

Technical
A Stability Factor of S = 1 is "marginal" where the bullet is verging on being unstable, so a Stability Factor of 1.5 is reckoned to be a good working value to ensure stability at low temperatures or high atmospheric pressures when the air density is high.

Inside the barrel, the bullet is rotating about its centre of form, but as it exits the muzzle it starts rotating about its centre of gravity. If the centre of gravity of a poor quality bullet is offset from the central axis of symmetry on which the centre of form lies, then there can be a sudden sideways jump as the bullet exits the barrel. This jump can lead to a short term instability resulting in a large yaw angle and nutation which is not good for short range accuracy. The faster the rate of twist, the worse this short term instability is and the bigger the group size. So historically it has been important to keep the Stability Factor as low as possible. These days however, the quality of bullet manufacture is so good that using faster twists, resulting in higher Stability Factors than S = 1.5, is not really a problem.

History
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.