Since the early days of firearm building, armorers noted that if they imparted spin to the projectile that it greatly enhanced in-flight stability and accuracy. The earliest rifles had numerous bands of metal that were forged together and twisted to create the helical shape of the rifle groves. As machining processes were developed and refined, hammer forged barrels became popular as they were much stronger and much more precise.
Rifle twist is represented with a “1;” a colon; and another number, such as 1:7, 1:9, 1:10, 1:12, etc. The second number is the length in inches that it takes for the grooves to make one complete revolution. Thus, a 1:10 twist rifle barrel makes a complete 360 degree revolution in 10 inches. A 1:7 rifle barrel on the other hand makes a complete turn in only 7 inches, giving it a much tighter faster rate of twist (and consequently a greater RPM to the bullet).
The Greenhill Formula, developed by Sir Alfred George Greenhill, lays out the mathematics for computing the optimum spin and rifle twist necessary to stabilize a bullet. His most basic calculation is
where C = 150 (or 180 for muzzle velocities greater than 2,800 fps) D = bullet caliber (in inches) L = bullet length (in inches) and SG = bullet’s specific gravity (10.9 for most lead bullets). For lead core bullets, the second half of the equation is disregarded as the value of the square root of 10.9/10.9 is 1, however the value will need to be calculated for steel core, steel jacketed, or frangible bullets as their specific gravity will vary. Because of the high muzzle velocity of most 5.56/.223 rounds, C should be set to equal 180 in the above formula.
What does all of this mean? For most shooters, not much. For our purposes, it means we can determine the appropriate twist based off of the bullet weight for a given caliber since bullet length is generally a function of the combination of bullet weight and caliber. Having said that, we’re not going to delve any deeper into the mathematics of calculating the optimal barrel twist for various bullet designs. Instead, we’ll lay out the basics and give you some good guidelines to go by when figuring whether or not your AR-15 barrel will stabilize a given round.
In general, you want a faster twist (lower second number) for heavier bullets. Firing lighter bullets through a fast twist barrel can over spin them, causing inaccuracy from overstability and/or spin induced drift. Overstability occurs primarily in light weight projectiles fired from a fast twist AR barrel and causes the bullet nose to remain at a high angle of attack during the descent phase of the flight trajectory, due to extreme gyroscopic stability. Extremely light weight, thin jacketed varmint rounds, that are overspun past 300,000 RPM, can even fly apart from the immense centrifugal forces imparted by the bullet spin.
For 5.56/.223 bullets weighing between 35 and 50 grains, you can use a 1:12 or 1:14 twist. 1:9 (probably the most common twist found in AR rifles) and 1:10 are good, moderate twist rates that are capable of stabilizing bullets weighing from 45 to 69 and even 70 grain bullets. For the heaviest 5.56/.223 bullets, you will need a 1:7 to 1:8 twist barrel in order to reliably stabilize bullets weighting between 69 and 90 grains.
There are some odd barrels out there being used to fire heavily customized .223 loads. Some custom barrels are available in a 1:6.5 twist and are capable of stabilizing 100 grain bullets, though that weight is not very common and difficult, if not impossible, to find. Extremely high velocity loads firing a bullet weighing 55 grains or less, at speeds exceeding 4,000 feet per second, require a very slow twist rate of 1:15 to 1:16.
Most shooters find that a 1:9 twist barrel meets their needs quite well, but if you’re going to be firing heavier match loads, or lighter, faster varmint rounds, you’ll need to search for a barrel with a more appropriate twist rate.