The M-16 A2 came with either full auto or 3 round burst, the A2 designation actually referred to the sights and hand guard and other minor changes and didn’t dictate the trigger group. The A3 simply added the removable carry handle and again was delivered with either full auto or three shot burst. The A4 added the railed handguard and the sights were changed from 8/3 to 6/3. Although I have not seen anything other than 3 shot variations I would assume that they (the A4) are made in full auto as well.