To build a AR pistol you have to start with a stripped receiver thats never been assembled into a rifle. thats the easiest way to start a pistol build. Luckily receivers prices are dropping after the panic buying from the long-legged mac daddy being elected. I'm building a pistol until I save up enough funds for a tax stamp, then I'm turning it into a SBR. I just cant decide what cal 5.56 or 6.8. I get most of info from AR15.com, just like this site the tech forums on there are full of great info.
http://www.ar15.com/forums/forum.html?b=3&f=122