Just to add, any preban Colt SP-1` or Government model AR will come very close to issued M16's, whether full size or the carbine models, A1 or A2, as they were issued. Current Colt AR's probably won't come as close to the military versions.
For a postban rifle, Bushmaster will be your best bet. They will have chrome lined chambers and barrels, military grade barrel steel, and all the internal parts will be milspec sized components (except the actual fire control and bolt assembly). You can get an A2, A2 car, or M4 variant from Bushmaster and then send it to Kurt's Kustom to have a fake flash suppressor or flash suppressor looking muzzle brake installed. You could even get a a filed bayonet lug on it so you will have the bayonet lug profile and not have it be functional.
Also, military carbines and m4's only have a 14.5" barrel. Civilian rifles have to be 16" in order to be legal (unless you have a full auto, then any barrel is legal). that means you have to ahve wither a 16" barrel or you can get the 14.5" barrel and have a muzzle attachemnt pinned and welded so that the overall length is 16".
You can have something extremely close or exactly like an issued M16, it just depends on how much money you want to spend and how much trouble you want to put into it. The beauty of the AR is that there are so many configurations. You could also buy an extra upper so that you can switch between a full size and an M4. The possibilities are almost endless.