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A rifle is a long gun with a barrel that has rifling (as opposed to a smooth bore). Rifling means the inside of the barrel has multiple grooves cut in a spiraling pattern to produce bullet spin when it is ejected from the muzzle. That bullet spin stabilizes the bullet in flight so it will more precisely hit a target at distance. The twist rate inside the barrel is what determines the rate of spin, measured by how many linear inches the spiral requires to complete one 360-degree rotation. The higher the twist ratio, the slower the spin rate; the lower the twist ratio, the higher the spin rate. No bullet flies perfectly, they all experience some "roll" (longitudinal axis), "pitch" (transverse axis), and/or "yaw" (vertical axis) such that the bullet in flight draws a kind of cone at its tip while it spins. But those imperfections typically make very little difference. We all know these basics, but what really is bullet stability and how is it measured?
There are two kinds of bullet stability: gyroscopic stability and dynamic stability. Following are very basic descriptions.
Gyroscopic stability (typically referred to as "Sg" for specific gravity) is the resistance of a bullet to changes in its axis of rotation. We all know what gyroscopic stability is from football. Throw a football without any spin and it will just tumble through the air, making it harder to hit your targeted receiver. But throw a football with sufficient spin and it will achieve gyroscopic stability, allowing it to stay on the line of axis intended by the quarterback, landing precisely in the hands of the receiver.
Remember that virtually every bullet has some tiny amount of instability when it leaves the barrel's muzzle. Fortunately, those elements of instability actually decrease during a well constructed bullet's flight due to dynamic stability. Generally, dynamic stability results from the stabilizing influence of air flow over the surface of the bullet during flight. In effect, air dampens bullet wobbling during flight. Needless to say, bullet shape will have a significant influence on dynamic stability.
The two most popular formulae for calculating bullet stability are the Greenhill Formula and, more recently, the Miller Formula developed by Don Miller. Of the two, the Miller Formula is more widely used today. The original Miller Formula had five factors: bullet mass in grains (called "m"); gyroscopic stability factor (called "s"); bullet diameter in inches (called "d"); bullet length in inches (called "l"); and barrel twist rate (called "t").
Miller developed his original formula around a standard bullet velocity of 2800 fps, which of course is not going to be as helpful for every shooter and the variety of bullets they use. So over time there have been many variations on the Miller Formula to get more specific bullet stability measures, depending on what you are actually shooting. One of the better ones, available for free and which allows specific velocity entries, may be found at http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi provided by JBM Ballistics. Enter the factors required on that web page, hit the "calculate" button, and a number will result. A number between 1.5 and 2.5 indicates sufficient bullet stability (1.5 to 2.5 is the bullet stability factor range used by the US military).
Here is an example. Let's say you want to evaluate stability for a bullet shot from a 300 PRC rifle with a 1:9 barrel twist. It is a Berger 168-grain VLD Hunting bullet (no plastic tip) that is 1.25 inches in length. The muzzle velocity is 3025 fps. Leave the default entries for temperature and air pressure as they are. The resulting calculation says the bullet stability factor is 2.924. You'll notice on the JBM website that the resulting number has a green background as an immediate indicator of good bullet stability. A yellow background, or worse a red background, are warnings of poor bullet stability.
Particularly if you are building your own rifle, you may find the JBM calculation for bullet stability, as well as their other ballistics formulae, of significant benefit.
There are two kinds of bullet stability: gyroscopic stability and dynamic stability. Following are very basic descriptions.
Gyroscopic stability (typically referred to as "Sg" for specific gravity) is the resistance of a bullet to changes in its axis of rotation. We all know what gyroscopic stability is from football. Throw a football without any spin and it will just tumble through the air, making it harder to hit your targeted receiver. But throw a football with sufficient spin and it will achieve gyroscopic stability, allowing it to stay on the line of axis intended by the quarterback, landing precisely in the hands of the receiver.
Remember that virtually every bullet has some tiny amount of instability when it leaves the barrel's muzzle. Fortunately, those elements of instability actually decrease during a well constructed bullet's flight due to dynamic stability. Generally, dynamic stability results from the stabilizing influence of air flow over the surface of the bullet during flight. In effect, air dampens bullet wobbling during flight. Needless to say, bullet shape will have a significant influence on dynamic stability.
The two most popular formulae for calculating bullet stability are the Greenhill Formula and, more recently, the Miller Formula developed by Don Miller. Of the two, the Miller Formula is more widely used today. The original Miller Formula had five factors: bullet mass in grains (called "m"); gyroscopic stability factor (called "s"); bullet diameter in inches (called "d"); bullet length in inches (called "l"); and barrel twist rate (called "t").
Miller developed his original formula around a standard bullet velocity of 2800 fps, which of course is not going to be as helpful for every shooter and the variety of bullets they use. So over time there have been many variations on the Miller Formula to get more specific bullet stability measures, depending on what you are actually shooting. One of the better ones, available for free and which allows specific velocity entries, may be found at http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi provided by JBM Ballistics. Enter the factors required on that web page, hit the "calculate" button, and a number will result. A number between 1.5 and 2.5 indicates sufficient bullet stability (1.5 to 2.5 is the bullet stability factor range used by the US military).
Here is an example. Let's say you want to evaluate stability for a bullet shot from a 300 PRC rifle with a 1:9 barrel twist. It is a Berger 168-grain VLD Hunting bullet (no plastic tip) that is 1.25 inches in length. The muzzle velocity is 3025 fps. Leave the default entries for temperature and air pressure as they are. The resulting calculation says the bullet stability factor is 2.924. You'll notice on the JBM website that the resulting number has a green background as an immediate indicator of good bullet stability. A yellow background, or worse a red background, are warnings of poor bullet stability.
Particularly if you are building your own rifle, you may find the JBM calculation for bullet stability, as well as their other ballistics formulae, of significant benefit.
