The Vietnam era M16 was chambered for 5.56 X 45 and shot a 55 grain full metal jacket bullet from a 1/12 twist barrel. False stories described the bullet leaving the bore tumbling end over end akin to a turning circular saw blade. Of course, it's rifling, imparting a spin on the bullet and velocity, keeps a bullet on a uniform path. In other words, if the bullet tumbled upon leaving the bore marksmanship would be impossible. Later, the M16-A2 was developed to correct and/or enhance the original M16's feature set. Among these was a 1/7 twist barrel which would stabilize a 62 grain bullet, as well as tracer ammunition. Although the 62 grain ammunition could be shot from the orginal M16, the 1/12 twist would not keep the bullet stabilized for shooting beyond about 200 yards. The M16-A2 also featured Bullet Drop Compensation Iron Sights, which permitted sight adjustment for a 300 meter battle sight zero and bullet drop compensation to extend sight adjustment to 800 meters. The M16-A2's .070 front sight post also serves as a range finding feature, whereby the E-type target will match the apparent width of the sight post at about 175 meters; thus, the shooter can better understand the need to apply bullet drop compensation. The M16-A2's rear sight adjustment value for elevation is 1 MOA per click, windage is 1/2 MOA per click. The front sight has a 1 1/4 MOA elevation value per click. Today's M16-A4 has a removable detachable carry handle BDC sight. Removing the carry handle permits an optical sight to be mounted, like an ACOG. Most types of ACOG's feature a range finding/bullet compensation reticle which permits very fast and accurate adjustment of aim to engage targets to 600 meters without need to re-adjust the ACOG's inital Zero to the latest target distance. Now, interestingly enough, from my experience in AR-15 sales, most buyers of the AR-15 are not interested at all in Marksmanship and therefore cannot shoot the AR to hit where aimed beyond a few hundred yards with the as issued sights or an optic. These "shooters" do not know anything about the principles of marksmanship, building a steady position, external ballistics, wind & weather counters. zeroing/MOA, shooter/target analysis, or range estimation; but, that's OK. They just need to understand firearm safety to assure good times with their rifle purchase and if hunting with the rifle the shooter needs to know the distance to which he can get a good hit on the target.