There is a lot of taper under the barrel so we are going to be using general numbers. Note I mentioned only 2 diameters per barrel. The typical AR has an HBAR barrel wich is heavier and less expensive to produce than the lighter weight government profile barrels. My M4 wannabe sports a Bushmaster M4 profile barrel and my 20" rifle has a M16A2 profile barrel (it was orginally an HBAR but I had Kurt's Kustom turn it down to the M16A2 specs).
In an M16A2 or M4 barrel (or even the M16A1) the barrel tapers down to roughly .60" just forward of the chamber to the front sight.
The civilan HBARS this area stays at roughly .98" (it varies from manufacture to manufacture but is at minium .9")
From under the front sight to the end of the muzzle ALL HBARS (civlian) and the M16A2 and M4 barrels are roughly .75".
M16A1 barrels taper down a bit finer (just under .60") to the muzzle
Now this is a ROUGH idea of what the barrels are profiled as. In reality its a bit more dynamic (i.e. I didn't take into consideration the step down for the flash suppressor threads) and the Military barrels are a bit smaller than the .75". The M4's M203 'cut' is roughtly .60" inches in diameter (same as it is under the barrel).
here is a picture of what the M4 barrel profile looks like:
www.bushmaster.com/shopping/barrel-assemblies/pbbl14m4aka.aspHere is a picture of an M16A2 barrel profile:
www.bushmaster.com/shopping/barrel-assemblies/abbl20a2.asp This is a great barrel and its lighter than an HBAR 16" barrel (by 3 oz).
Note how they taper down after the chamber and taper up just before the front sight.
Now here is an HBAR 14.5:
www.bushmaster.com/shopping/barrel-assemblies/abbl14.5a.aspNote after the chamber there is a 'slight' diameter change down to .98", which steps
down to .75" just before the front sight (note the M4 barrel ramps
up to the .75" portion).
Here is an HBAR 20":
www.bushmaster.com/shopping/barrel-assemblies/pbbl20a.aspThe HBAR 20" barrels are a full pound heavier than the Govt profile M16A2 barrels.