[L473ncy]

To start AR15 is the Armalite model 15 designed by Eugene Stoner.

ARXX just means the "model" that it is. So there's things like AR5, AR10, AR18, etc.

Armalite sold the rights to the AR15 (and 10?) design to Colt and Colt ran with that and the US Army purchased it.

So every M4/16 or whatever is based on the AR15 platform (eg. offerings from Olympic Arms, Bushmaster, Norinco, DPMS, Sig, Mossberg, S&W, HK etc.). I don't know of any "exotic" AR's (ie. not based on the AR15 platform) in the airsoft world but I'm sure people could custom make something like an AR10 if they were so inclined.

So for all intents and purposes in the airsoft world M4/16=AR=SR (Stoner Rifle which I believe Knights Mfg owns the rights to). I'd say it's a very "modular platform" and you can change just about anything and keep it the same like barrel length from 7 inch AR pistols to big 20 inch long range hunting/match rifles.

I would recommend starting with a standard 14.5" AR and going from there. As for brands do some research but VFC makes nice models, G&G Regular/TopTech line, CA X-Series (not the old series), as does G&P (with the caveat that you need to strip it and reshim the gears properly or replace the gears entirely). As far as fitment issues go as long as you get a "G&P style" body where the upper has "sliding tabs" where the upper and lower connect you'll be fine. As opposed to the CA "2 pin system" of which I lost one of the pins from my Madbull body (which I believe all use the CA 2 pin system) and I'm really considering turning a single pin on a lathe because it annoys me lol.

And Thunder Cactus is wrong, there is a reason to have the longest barrel you can. I know that Ricochet loves having the longest barrel he can but it's probably also because he's over 6 ft. tall. I don't think he actually has anything shorter than a 14.5" barrel on his guns and the first time I met him I think he had a 18" barrel on his PTW.