http://world.guns.ru/assault/as18-e.htm
" 1981. Colt developed a variation of the M16A1, adapted for the SS109/5.56mm NATO cartridge, and submitted it to the military trials as the M16A1E1. This rifle differed from the M16A1 by having the heavier barrel with faster 1:7 rifling, a different type rear sights ( adjustable for both range and windage), round handguards instead of triangular ones, and by replacing the full-auto fire mode with the burst ( limited to 3 rounds per trigger pull), to preserve the ammunition.
1982. M16A1E1 is type-classified by US DoD as the " US Rifle, 5.56mm, M16A2"."
En gros le M16a1 a fait l´objet de plusieurs modifications incluant notament l´abandon du fulle auto pour le burst, cela donne le M16a2.