I competed in the Oregon National guard during most of the 1980's
We had M 16 A1's (1 in 12" twist) We tested several trying to find sweet ones. We use a colt scope attached to the carry handle and fired M193 ammo (55gr fmj) from a bench at 100yds.

Most shot 1 1/4 to 1 1/2 inch 5 shot groups, On occasion we would get a 1" group but the rifle would seldom duplicate it. I think that the A1 with it's 1 in 12 twist was the most accrate of the M16 series with the 55gr m193, the M2's did not seem to group as well with the 55 or 62gr bullet. I don't know about any of the results with 75 and 77gr bullets in the latter m16 series

My own personal results run from 1 to 2 inches with various AR's and my better ones shoot 1 moa or less with 69gr sierria match kings.
my bushmaster preditor has shot .5 to .6 with the 69's almost on demand.
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he went into younder village and never returned