Shooting accuracy comes from good technique, not a particular type of weapon.
Good optics will help you see your target, but not hit your target.
In addition, the M16 (A2 when I was in) is only accurate out to about 500m for a man sized target. If you are really trying to hone your technique then you will need to attempt further distances, which means you will need a bolt-action rifle. Remington 700 gets my vote. Not only is that design inherently more stable, you can purchase a better chamber for long range shooting. The 5.56 just won't cut it. A .308 or .30-06 would both be good for distance, easily handled, and both rounds can be easily found and purchased.
If you're insistent on a AR type weapon I can't offer much advice. I don't do a lot of match shooting and junk. However, keep your mind open to alternatives. The AR-15 can be a very quality weapon, but it's not always the best choice.