This has been discussed before, but the Army didn't agree with the A2 barrel profile. From their 1986 study:
"The M16A2 "heavy barrel" is heavy in the wrong place. The problem with the M16A1 is a temporary bending of the barrel which occurs from the stress of various firing positions causing bullet strike to vary, e.g., the difference between a bipod firing position, and a position using a hasty sling will change the strike of the bullet at 300 meters by three to four feet or more. The "bending" takes place between the receiver and the sling swivel/ bayonet stud. The M16A2 barrel is "heavy" only from the sling swivel to the muzzle--where it can have no effect on the bending problem."
In fact, the Army apparently found very little to like about the M16A2. Maybe they didn't care for the fact that it was a USMC initiative:
https://apps.dtic.mil/dtic/tr/fulltext/u2/a168577.pdf