The differences between the A2 and A3 sights are that the A2 sight is installed directly into the receiver while the A3 sights are a part of a detachable handle. The sight base in the detachable A3 handle is shorter. The A2 sight is adjustable for 1 MOA elevation and 1/2 MOA windage. MOA ( minute of angle ) is 1" at 100 yards. The A2 elevation adjustment is good for out to 800 meters with an 8/3 index. The A3 sight is adjustable for 1/2 MOA elevation and windage with the elevation adjustment out to 600 meters with the 6/3 index.
Since the A3 handle is 1/2 MOA elevation adjustment you need to click up 2 clicks instead of 1 to the "Z" mark to set for 25 meter battlefield zero.