556 NATO AR15 Barrel 

The military-issue M4 carbine is chambered in 556 NATO with a 14.5" M4/GOVT contour 1x7 twist barrel, and the classic civilian AR15 is a 16" 556 NATO configuration.  A 1x8 twist will stabilize all factory loads unless you plan on firing tracers which was the military's reason for choosing 1x7.  556 NATO ammunition is readily available, inexpensive, and effective in both pistol and rifle applications.  AR15Parts.com stocks a wide range of 556 NATO barrel lengths in a variety of finishes.

AR15Parts.com barrels are sourced from top US manufacturers including Ballistic Advantage, Shaw Barrels, Swan Machine, Wilson Arms, Wilson Combat, ZRO Delta, and others.