[EricCartmann]

Actually they are all AR-15's. The Army always like to designate stuff M-something. The Abrams tank is called a M1. The Humvee is called a M998.

Same goes for the AR-15. Army calls them M16's. I guess technically if you have a full auto AR-15 then it can be called an M16 or M4.

M16's were the orginal AR-15's with 20" barrel.

M16A1's were improved versions with forward assist and other stuff. Both the M16 and M16A1 were select fire in either fully auto or semi.

M16A2's replaced the M16A1's, had a rear site that was adjustable for elevation, a different twist rate to handle M855 and M856 ammo, and some other stuff. M16A2 were select fire weapons with 3-round burst or semi option. But the M16A2 I was issued when I was in the Army did not have an adjustable elevation rear site. Only when we were issued the newer A3's is when I remember the site being adjustable for elevation.

M16A3's are basically M16A2's but with full capabilities instead of 3 round burst.

M16A4's have a flat top upper with removable carry handle and railed handguards.

M4's (3 round burst) and M4A1's (full auto) are basically M16A2's with a shorter 14.5" barrel, collapsible buttstock, and flat top upper reciever.

Most of us have what you would call M4-forgeries. Exactly as an M4 but in semi only and with 16" barrels (to conform with Federal Law). 14.5" or shorter barrels are legal if you did the paperwork and paid the $200 tax stamp. Also if you have a permantly attached (pinned and welded) FH that brings the overall length to 16" or more, then you don't have to pay the tax stamp.