Building a standard AR-15 pistol is practically illegal. You might be able to find a pistol-registered pre-ban stripped (i.e.: documented proof it was a complete AR-15 pistol at one point before the ban) receiver, but they're rediculously expensive. I looked into it shortly after the ban, and the few bare receivers left were going for about $1000 (you can buy a complete & legal Carbon-15 Pistol for that).
That said, you can get/make some interesting variants. Go to
http://www.ar15.com - there's usually a few discussions going. There are carbon-fiber versions and swiss-cheese versions that are under the 50 oz limit. Some people are building bolt-action versions: some with a straight-pull left-side bolt handle, and some using the charging handle as the bolt handle. Some weld the magazine in place and load it through the chamber. At least one guy is making what he calls the "auto-unloader" (apparently have to manually release the bolt after each round fired).
I really wanted to make one from scratch, just as you apparently do, and didn't want anything less than a "true" AR-15 pistol. The only legal way was to get a hideously expensive properly-documented registered receiver. Unless you want to fight the "assault weapons ban" (be my guest!), I suggest you find some other unique gun to build; a post-ban AR-15 carbine is a close option, and ironically a short-barreled rifle (barrel under 16") is more legal (just jump thru the BATF hoops, beware of state restrictions), cheaper (just a $200 tax), and more useable.
