There are a selection of AR / M16 ammo types. The 5.56×45 at top and bottom, the 7.62×51 (.308) second from top, 6.8 SPR and 9mm below that. From a few feet away they look pretty much the same, as you look more closely the differences are apparent in the magazines. Their stocks are all much the same, except the SBR which is sporting the Vltor Clubfoot Mod Stock. There are some cool angled shots of this selection of AR15 / M16 carbines:
[Via kill a computer.com]
Related posts:
M16 – United States Assault Rifle
Colt M4 – Versatile Carbine