In 1980, NATO approved the adoption of the 5.56 x 45 mm  NATO cartridge as the second standard caliber, and the Belgian SS109 as the basis for standardization of ammunition. The SS109 bullet design utilizes a 62 grain Full Metal Jacket Boat Tail (FMJBT) projectile. To achieve proper spin stabilization of that long, heavy bullet, a fast twist rate was required. Barrels with a 1:12” riffling twist offer best accuracy with bullet weights from 45 to 55 grains. Although bullets from 40 to 60 grains can be stabilized, accuracy will not be optimum. The common 1:9” twist remains optimum for bullets weighing between 55 to 70 grains. Although lighter bullets can be used in a 1:9” barrel such light weight bullets may be over stabilized and accuracy may suffer. A 1:7” barrel will offer best accuracy with heavier, longer bullets from 68 to 80 grains. Accuracy with these fast twist barrels with lighter bullets is poor.

As noted there is an over lap between the various barrel twists. With typical military surplus and factory ammunition being between 52 and 62 grains the common 1:9” barrel has the most utility. Those wishing to compete in long range target shooting where the heavy 70 grain bullets are hand loaded to lengths that won’t fit in a standard magazine the faster 1:7” twist rate will stabilize those bullets for the most accuracy.

7 Bolt Carriers sm
AR-15 Twist Rates