Given the same sights and shooter, what would be expected in accuray at 100 to 200 yards between a Bushmaster 16" barrel carbine and a colt target match 20" HBAR ?
Not much difference between either. Using aperature sights the 20" should be more accurate because of the longer sight plane. I'd buy the cheaper one or the one I wanted of the two because realisticly you probably will not see any accuracy difference.
The 20" has a sight radius advantage. If both rifles had the same scope, the Hbar may have a slight edge. You would need to be a very good shooter to see the difference.
I figure that any old basic AR should be capable of around one to two MOA. The target versions are generally capable of around a half-MOA.
Achieving this depends on the shooter's capability, particularly as to being very consistent for sight picture when the trigger is pressed. That consistency is the key to all shooting, regardless of type of rifle or type of sights.