I find the three AR-15's I have dependent on the ammo to dictate accuracy. I have a S&W AR 15 Sport II and two PSA AR -15's. The PSA AR's have a 1/7 inch twist barrel where the S&W has a 1/9 inch twist. Mine seem to be in the 2 MOA at 100 yards.
I know this is an older thread, but will share my experience. Straight out of the box, iron sights, standing un supported at 100 yards, consistent 4" groups. I don't know about anyone else, but I was very satisfied. Especially, considering my RRA that cost almost 3 times what the Smith did gets 2" groups with a red dot.
My 16" S&W improved significantly when I added a 4 lb. Timney trigger.
With the standard GI trigger, the whole gun shakes when the hammer hits. With the Timney trigger (which is self contained in a 'cartridge'), the gun does not shake. The cartridge design absorbs the vibration instead of transferring it to the receiver.