If you go with the old, proven Cav Arms lower, it's a pound lighter than a comparably built aluminum lower. Cav Arms isn't producing them anymore, but a newer company picked up the rights, and has them in production again. I'd start there, first and foremost, on a sub-6lb build. That's an easy pound of it, right there.
http://www.gwacsarmory.com/lower-receivers-1/