![]() 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. 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. There are some odd barrels out there being used to fire heavily customized. For the heaviest 5.56/.223 bullets, you will need a 1:7 to 1:8 twist barrel in order to reliably stabilize bullets weighing between 69 and 90 grains. 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. Extremely lightweight, 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.įor 5.56/.223 bullets weighing between 35 and 50 grains, you can use a 1:12 or 1:14 twist. Overstability occurs primarily in lightweight 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. Firing lighter bullets through a fast twist barrel can over spin them, causing inaccuracy from overstability and/or spin-induced drift. ![]() In general, you want a faster twist (lower second number) for heavier bullets. Because of the high muzzle velocity of most 5.56/.223 rounds, C should be set to equal 180 in the above formula. 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. 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). 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. 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). Thus, a 1:10 twist rifle barrel makes a complete 360-degree revolution in 10 inches. The second number is the length in inches that it takes for the grooves to make one complete revolution. Rifle twist is represented with a “1 ” a colon and another number, such as 1:7, 1:9, 1:10, 1:12, etc. As machining processes were developed and refined, hammer-forged barrels became popular as they were much stronger and much more precise. The earliest rifles had numerous bands of metal that were forged together and twisted to create the helical shape of the rifle groves. 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.
0 Comments
Leave a Reply. |