The following information is important especially if you are reloading rifle ammunition. However, the brief blog just scratches the surface and additional research and field testing is required.
Rifling is the lands and grooves inside the barrel that provide spin on a bullet as it travels down the bore. This spin stabilizes the bullet in flight similar to a football spiraled by a quarterback.
A barrel's twist indicates how far the bullet must travel through the bore to make one full revolution. This is the rate of rotation of the rifling. For example, a 9" twist barrel spins the bullet one full turn within 9 inches. In a 16" twist barrel, the bullet makes one revolution in 16 inches. So the 9" twist is faster (spins the bullet faster) than the 16" twist. Seems obvious but it's worth stating that the 9" twist is less twist than a 16" twist.
The twist in a rifle barrel is designed to stabilize bullets. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity (same caliber) longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight. Heavier bullets require a faster twist than lighter bullets of the same shape.
In most guns, muzzle velocity is determined by the quality and quantity (burn speed, expansion) of the powder, the mass of the bullet, and the length of the barrel. A slower burning powder needs a longer barrel to burn completely, but can use a heavier bullet. A faster burning powder may accelerate a lighter bullet to higher speeds if the same amount of powder is used. A balance between powder quality and quantity, bullet mass, and barrel length determine optimal performance and safety.
If you are reloading your rifle ammo, you want to know the twist rate of your rifle(s) to ensure the correct bullet weight is used. Most AR-15 rifles have the twist rate printed on the barrel - sometimes up close to the muzzel end. If you can't find it, it's probably identified in the manual and / or the manufacturer can be contacted to find out or verify the twist rate.
There are websites that provide charts that help pull this information together. One such website is Shilen (http://www.shilen.com/calibersAndTwists.html). Listed are general twist guidelines by caliper. For example for .224, a 7 inch twist is good for bullets heavier than 70 grains; a 9 inch twist is good for bullets up to 70 grains.
Berger Bullets has a Twist Rate Stability Calculator at:
http://www.bergerbullets.com/twist-rate-calculator/
This Calculator is specific to Berger Bullets and there are 3 categories possible: Unstable, Marginally Stable and Comfortably stable. There are several paramaters to identify including bullet, temperature, altitude, barrel twist, etc. Once the parameters are entered the stability is displayed.
Field Test
I tested two AR-15 type rifles with a 9 twist using several different bullets. Bullets used:
- Hornady V-Max, 50 grain
- Hornady Varmint, 55 grain
- Nosler 77 grain
- Nosler 68 grain
- Berger 73 grain
Powder: Hodgdon Varget
Both rifles were most accurate with bullets below 70 grains.