A few other additions:
- There are really 3 categories or names for the guns in this family - the M-16, the M-4, and the AR-15.
M-16's are always military and have full auto (M16A1 and A3) or the 3 round burst (M16A2 and A4) function. You can't have one unless you steal it or are rich. M-16s have a 20" barrel.
M-4's are also military, have the 3 round burst (M4) or full auto (M4A1) option, but have a 14.5" barrel. They are easily distinguished due to the notched barrel a few inches from the barrel. It's used for connecting the M203 grenade launcher.
The M-4 model is also available to civilians without selective fire. Guns with a barrel less than 16" are categorized as SBRs (Short Barreled Rifles) and you have to pay big bucks to get one. As such, most civilian M-4s have a 1.5" flash hider and/or muzzle brake to extend the 14.5" to 16". There's much debate as to whether these differences still makes the gun an M-4 or something else. Thus was coined the term "M-forgery". The short of it is that you can buy an M-4 with these minor changes.
AR-15s is a term is used to describe all civilian guns that fit in the categories and others. A non-military M-16 is an AR. As is a civilian M-4. Or a .308 Bushmaster. AR's come in many different calibers, styles, and configurations. M4's and M-16s are always 5.56/.223.
- While most ARs can interchange 5.56 and .223, some cannot. Make sure your gun can fire both before trying it out. You must check the inscription on the barrel for this, not the lower.
- There are generally three types of uppers - A1, A2, and A3/A4. A1's and A2's have a permanently attached handle and rear sights.
Optics are attached onto the carry handle putting them several inches above the barrel.
A3's and A4's have removable handles so you can attach optics, etc. to the picatinny rail on the top of the gun. All M4's have flat tops.
This, when used in conjunction with hand guard rails, allow uncountable configurations of optics, attachments, dohickies, and thingamabobs.
The biggest advantage to the flat top is that you can swap optics with negligible impact on accuracy. (Technically speaking, an M-16A3 is also a fixed handle, but for civilians A3 usually means flat topped.)
- Regarding brands, there's not a significant difference between most manufacturers. Colt, DPMS, Bushmaster, and Rock River Arms always seem to be the most highly reviewed. And the beauty of ARs are the part are usually interchangeable - meaning you can have uppers, lowers, and other parts all from different manufacturers. As such, the key it to find manufacturers that most closely build them to spec tolerances. I've been very happy with my Bushmaster M4
(also at Impact Guns