D. Barrel Twist Rate
The twist rate of the barrel is the rate of turn the bullet will experience in a given distance. For the purposes of this article only the 223 Remington / 5.56 NATO rounds will be discussed. The information provided will prove a point which can then be applied to any of the nearly countless AR15 chamberings.
First, the twist rate, or ratio, can be defined using “1:7” as an example. What this means is the bullet will experience one complete revolution within a 7” length. Whereas a 1:9 twist rate means the bullet will experience a complete revolution within 9” of length. Which is right? They are both right depending on the needs of the user, and the intended ammunition to be shot from the barrel.
The higher the twist rate, meaning the lower the number of inches taken to complete a revolution, means the more stable the bullet will be leaving the muzzle of the barrel. However it is possible to over stabilize a bullet. For example shooting a 40 or 50 grain varmint round from a 1:7 barrel may cause the bullet to turn at such high revolutions per minute (RPM) that the copper jacket can actually be removed from the lead core. This over stabilization can cause a major loss in accuracy and a loss in ballistic performance for hunting purposes. This is why it’s important to match the barrel to the ammunition, and visa versa.
When it comes down to it, any twist rate will shoot just about any bullet, but when discussing optimization the twist rate should be matched to the ammunition you intend to shoot. The general rule of thumb used is that a 1:7 will shoot bullets in the 70 grain range the best, but will also shoot 60 grain bullets just fine and even most 50 grain bullets without any problems. 1:9 shoots varmint bullets in the 40 and 50 grain range the best, but will also shoot heavier bullets, though at longer distances the heavier bullets will become unstable and cause a loss of accuracy. 1:8 seems to be the modern trade off and is a twist rate gaining popularity. The 1:8 seems to shoot everything well. So the barrel should be matched to the bullet, and if you are not sure; you usually can’t go wrong with a 1:8.
E. Barrel Length will affect many factors on the flight of the bullet including velocity and distance. All things being equal a longer rifle barrel will generally produce more accuracy at longer distances due to the increased velocity, than a shorter pistol barrel for example. It is also important to note that barrels shorter than the carbine length 16” are sometimes considered to be controlled items by the ATF and special permission is required. One must also check short barrel rifles (SBR) with their state, county and city as SBR’s are illegal in some areas.
14.5” barrels are controlled, therefore not owned by most civilians. Though a 14.5” barrel with a permanently attached long muzzle device will usually satisfy laws regard Short Barrel Rifles(SBR). 14.5” barrels are absolutely great for CQB or urban tactical and home defense (with the proper permission). The trade off is a loss of accuracy and terminal performance at longer distances.
16” barrels are the most popular allowing for the most flexibility within the rifle, especially because they do not require any special ATF permission slips. 16” barrels are seemingly good at everything, home defense, three gun competition, most tactical situations, and will even reach out with combat accuracy and effective terminal ballistics to 300 yards or so.
18” barrels are gaining popularity. These barrels are especially popular with three gun shooters. They are usually coupled with mid length or intermediate gas systems giving a great balance of distance, velocity, and reduced recoil impulse while maintaining a manageable weight.
20” barrels were used on the original M16 rifles (the grand father of the modern civilian AR15). The longer barrel will give you more velocity which will increase the effective range of the bullet. Bench rest shooters may desire this increased accuracy thus opt for a longer barrel. However the added weight of the barrel may be a turn off for hunting or tactical purposes and may be clumsy for use in home defense or CQB.