I'm doing an AR Build right now. You will be much happier if you buy a good quality matched receiver set. While you can put just about any upper on any lower, it will never look as tight as a quality matched set. And, if you are taking the time and money to put together your custom AR, aesthetics do matter. Especially if you ever intend to try to get your money out of it.
The way a billet of 7075 is produced is essentially the same thing as forging. The billet is not made from a casting. There is a good reason to look into high quality billet 7075 upper receivers - this is if you are going to free-float your barrel. Free-floating a barrel put additional stress directly on the upper receiver, especially if it gets knocked around. The billet uppers usually add material near where the barrel mounts to strengthen it. After researching this, I decided to go with a matched billet set made from 7075 - as I do intend to free-float the barrel.
Last edited by Skans; February 5, 2013 at 06:01 PM.