In 1977 - 80, NATO conducted a series of tests to settle the issue of which cartridge would replace the 7.62-mm NATO round as its future cartridge. The 5.56-mm had been out for a couple of decades and was considered a favorite, but an ammunition manufacturer in Belgium had an idea. They developed a 5.56 round, the SS109, that was longer and heavier than the US M193. The newer round, if adopted, required a different twist of rifling. Colt was approached and asked to look into the new requirements. They were able to keep much of the successful M16Al design, but they added a heavier barrel. They changed full auto to a three-round burst selector. The rear sight has been improved, as has the flash suppressor (no bottom slot), and a brass case deflector helps to direct hot brass away from left-handed firers. (This was the best addition for me since I shoot left-handed). A chronic problem with the old M16 was that the hand-guards would pop off. A re-designed hand-guard corrected this with the M16A2.