the old 1-12" barrels could be very accurate. i can recall many an a1 that could put them all in the same hole in the short range target.
long range --we did have much in the way of optics with the old a1s m by the time the ACOG type things came around the a1s were for the most part stood down from front line use.
so i don't know what kind of group they could do @ 500M+ , i do know that knocking the targets down on the target machine wasn't difficult at 500M with the a1 peeps.
i don't think that the old barrels were as "steady" as the newer ones, that could have been that we abused the old m16s more-- like when the got hot they would open up some. nothing like getting a barrel red hot shooting thousands of blanks full auto, or even worse firing hundreds of tracers full auto --so it ain't like we were babying the things to get a sum moa accuracy out of um--lol
the ammo that your shooting plays a big role in how accurate a rifle is as well. we had some very good m193 ball powder ammo back then that was very stable velocity wise.
no that ain't me in the pick --that my old pal "big-kinfe" billy--