Omega Alpha

Comparison between clones.  Top is an M16A2, bottom is an XM16E1.  Being the last and first M16's issued by the US Army.  The XM16E1 replaced the M14; the M16A2 was replaced by the M4.  Between them is the M16A1, which I don't have a clone of handy.

Edit:

The evolution from EXM16E1 to M16A2 has many changes you can see here.  The stock gets longer by about 5/8", pistol grip changes contour, handguards change shape; all the furniture is made from better, more durable materials.  The rear sling swivel is relocated and made fixed.  The front swivel is retained by a rivet instead of a roll pin.  A brass deflector is added behind the ejection port.  The ejection port door gets a different pad on it that prevents it from binding on the lower when the action is opened up on the front pivot pin.  The magazine release gets a fence put around it.  Several areas of the lower receiver are beefed up.  The shape of the forward assist changes.  The rear sight goes from a simple two position flip to a 300-900 national match ready unit.  The flash hider goes from a three prong to a partial bird-cage that's solid on the bottom.  The barrel is of an increased diameter from the handguards forward.  On genuine military guns the full auto setting is changed to a three round burst.

It also goes from being a 6.8 lb. rifle to a 7.2 lb. one without magazines.  20 rounders add 0.6 lb. and 30 rounders add 1 lb.  Nearly all of that 0.4 lb. is out front and it makes the rifle considerably less handy.