Some folks have mentioned that there are ACOG replicas out there that only look good, & have no adjustment. For those posing picutures I guess.
So take it off the rifle. Set it down on a flat surface and take note of where it's sitting and where the crosshairs are. Take the dust covers off the adjustment towers. Get a coin or tool or whatever and wind both dials all the way in and out a few times. While you do that contine to take note of where the crosshairs are and if they've moved.
Then go back to your rifle, at about 50-60 feet sight in with the irons. Then put the ACOG back on and zero it if yours is an adjustable model.
Airsoft, where nothing is hurt but feelings.