Barrels

The barrel is the component with the largest single influence on accuracy, and the specifications that matter are twist rate, length and chambering. Twist rate stabilises bullet weight — a 1:7 twist handles heavy 77 grain projectiles that a 1:9 will not, while a 1:9 shoots light bullets very well.

Length is a trade between velocity and handling, and the returns diminish quickly: past about 20 inches on a 5.56 you are adding inches for very little speed. Chambering is a safety matter, not a preference — a barrel marked .223 Remington should not be fed 5.56 NATO. Fitting a barrel is gunsmithing on most platforms; the AR-15 is the notable exception.