Mil-spec parts, the company name dont matter unless we are talking about barrels, and even then it dont really matter. So get mil-spec
5.56mm chamber, if it says 223 or 223 Wydel then avoid it. It wont reliably cycle mil-spec ammo, there is a chamber differance.
Chrome lined, Im going to say that if you ever really intend on abusing your rifle, or belive it will spend time in wet enviroments then get the chrome lineing, if not then dont sweat it.
Dont get any sort of optic untill you can use the irons like it was second nature.
M4 generaly means that the barrel is 14.5in(meaning that the flash hider must be NON removable), That it has larger feed ramp cuts in the barrel extention and reciver, that the barrel has a profile to allow an M203 grenade launcher to attach, and the extending stock is larger and better fitting then the original CAR stock. The feed ramp cuts and the stock are the only opperational improvments. You can get these improvments on pretty much any AR15 you would like.
Im also going to sugjest that you build the gun yourself from a compleat parts kit. You will need to only buy the lower reciver from an FFL the rest can ship to your door. If you do this correct you can build an AR15 for less then 600 bucks. Not only will you get a better price but you will know exactly how it works, and be able to fix it if the need arises.
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