The M16A4 ACOG is an M16 variant equipped with an ACOG scope for medium range engagements. It is quite rare compared to the other M16 rifles because it can be found only in the barracks at the northwest airfield, a place highly trafficked by hostile players.
The M16A4 ACOG is best at engaging targets at ranges of 0 to 400 meters. Rapidly firing in burst-fire mode makes it indistinguishable from a full-auto weapon for CQC, and the scope provides accurate shots at ranges up to 600m. One thing is key to using this rifle effectively when sniping - aim for the head. This rifle can get consistent head-shots at ranges <500m. This weapon isn't as good at CQC as an AKM, and it isn't as good at sniping as a DMR, but if you play with a one-weapon loadout, the M16A4 ACOG is likely the best all-around option in DayZ.
As with any other weapon in DayZ, using an SD magazine makes the weapon virtually silent to zombies. Bear in mind, however, that the actual sound that players hear is still just as loud as with a normal magazine. Also, the SD magazines change the default zeroing in an un-suppressed weapon to 1000m, meaning to get a headshot at 100m, you will have to aim for the waist.
When zoomed, these are the ranges:
Center Line: 0-100m
1st Line: 200m
2nd Line: 300m
3rd Line: 400m
4th Line: 500m
Final Line: 600m