The differences between the A2 and A3 sights are that the A2 sight is installed directly into the receiver with fixed carry handle while the A3 sights are a part of a detachable handle used on the flat top receivers with Picatinny rail. 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.
Only the A3 models have removable carry handles on the flat top receivers with Picatinny rail.
All Bushmaster 5.56/.223 barrels are chambered in 5.56mm so both 5.56mm and .223 ammunition can safely be used. SAAMI does not recommend firing 5.56 ammunition in rifles chambered in .223 as the shorter throats will cause higher than normal pressures.