I am wondering if anyone can point me to a source that explains the reason behind the heavier barrel profile the military used on the A2 model.  I know that stiffer barrels are usually more accurate, but the A2 HBAR is only larger in front of the front sight.  Under the handguards it is as thin as the A1.  I would think that increased stiffness at the base of the barrel, where it joins the receiver, would reudce barrel whip more than a thick muzzle.  I guess the heavier muzzle might act as vibration dampener and even help to reduce muzzle flip.  It just seems strange to only make the barrel larger on the end.  Why didn't they up the barrel diameter by a lesser amount along the entire length?  I have heard all kinds of stories about bayonet use, opening cans, grenade launchers, etc. I would like to see something quasi-official that explains it all.  

A2 and HBAR are different;

A2
[img]bushmaster.com/shopping/barrel-assemblies/Images/abbl-20a2a-small.jpg[/img]

HBAR
[img]bushmaster.com/shopping/barrel-assemblies/Images/abbl-20a-small.jpg[/img]

I am aware of the the difference.  I am inquiring about the "Government" profile A2 barrel.  I thought the military referred to this as the HBAR, also.  The commercial HBAR's are, of course, as you showed in the picture.

 The A2 profile was left in the standard configuration under the hanguard in order to accomodate the M203 grenade launcher.

The Colt A2 HBARS preban are REALLY fat under the handguard.