Twist rate
Twist rate is a firearms term that refers to the rate of twist of a gun barrel's rifling grooves.
It is usually expressed as a ratio of one twist per n inches or centimeters.
For example, a 1:7" twist rate means one complete rotation in 7 inches (180 mm).
Twist Rate ([1])
Bullet stability depends primarily on gyroscopic forces, the spin around the longitudinal axis of the bullet imparted by the twist of the rifling. Once the spinning bullet is pointed in the direction the shooter wants, it tends to travel in a straight line until it is influenced by outside forces such as gravity, wind and impact with the target.
Rifling is the spiral or helix grooves inside the barrel of a rifle or handgun. These grooves were invented a long time ago, perhaps as early as the 14th century. However, the smooth bore, using the round ball, was the choice of weapons for warfare even through the American Revolutionary war. The smooth bore musket could be loaded faster than the rifle and didn’t foul as badly with the combustion products of black powder.
The rifling grooves helix is expressed in a twist rate or number of complete revolutions the grooves make in the corresponding barrel length. The ratio is usually normalized to one revolution. For example, a 1-in-10 or 1:10 ratio indicates one complete turn in 10 inches (254 mm) of barrel length; 1:7 indicates one turn in 7 in. (~178 mm).
How important is twist rate? David Tubb, a winner of several NRA High Power Rifle Championships, was using a .243 rifle with a 1 in 8.5 twist barrel. He wasn’t able to get consistent accuracy until he changed to a rifle barrel with a 1 in 8 twist. The ½ in (13 mm) twist change made all the difference between winning or losing the match.
A term used to describe twist is "overstabilization" of the bullet. Faster than optimum twist does tend to exaggerate errors in bullet concentricity and may cause wobble. The faster twist also causes the bullet to spin at higher rpm, which can cause bullet blowup or disintegration because of the high centrifugal reaction forces generated. For example, the .220 Swift, at 4,000 ft/s (1200 m/s), spins the 50 grain (3 g) bullet at 240,000 rpm.
One of the first persons to try to develop a formula for calculating the correct rate of twist for firearms, was George Greenhill, a mathematics lecturer at Emmanual College in Cambridge, England. His formula is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers. This can be simplified to:
Twist = 150 X D²/L
Where: D = bullet diameter in inches L= bullet length in inches
This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. The twist formulas used in the Load From a Disk program, featured at this web site, uses a modified Greenhill formula in which the "150" constant is replaced by a series of equations that allow corrections for muzzle velocity from 1,100 to 4,000 ft/s (300 to 1200 m).
The Greenhill formula is simple and easy to apply and gives a useful approximation to the desired twist. The Greenhill formula was based on a bullet with a specific gravity of 10.9, which is about right for the jacketed lead core bullet. Notice that bullet weight does not directly enter into the equation. For a given caliber, the heavier the bullet the longer it will be. So bullet weight affects bullet length, which is used in the formula.
To measure the twist of a barrel, use a cleaning rod and a tight patch. Start the patch down the barrel and mark the rod at the muzzle. Push in the rod slowly until it has made one revolution, and then make a second mark on the rod at the muzzle. The distance between marks is the twist of your barrel.
To see how this works out, assume you bought a .222 Remington rifle and you measured the twist rate as described above. The twist was 1 in 14. You have two .224 bullets you want to use, the 70-grain (4.5 g) Speer SPS and the 50-grain (3.2 g) Hornady SX. The Speer bullet measures .812 inches in length and the Hornady measures .520 inches. Using the formula above we calculate the following twist rate:
Speer 70-grain (4.5 g): 1 in 9, Hornady 50-grain (3.2 g): 1 in 14
These calculations show that the 50-grain (3.2 g) bullet will be stabilized, but the 70-grain (4.5 g) won’t. Sure enough, when you try these bullets out, the 50-grain (3.2 g) shoots ¾ MOA while the 70-grain (4.5 g) won’t group on the paper at 50 yards. Twist is important!
[edit] Further reading
A very interesting read about bullet design and stability that is more complex than the Greenhill formula is What is the Maximum Length of a spinstabilized Projectile? by Mr. Beat Kneubühl
