If you're just taking things out of the trash bin and bolting them together to see what you get, that's something different than what I gathered from your first post.
If I were looking to build a lightweight AR from scratch, I'd still start with a quality Barrel and BCG.
This barrel from Daniel Defense gets my vote, as does
this BCG from BCM. I'd put an M4 stock, A1 pistol grip, oval mid-length handguards, and irons instead of optics before I'd scrimp on the barrel and bolt.
If you need the FSB, gas tube, and barrel nut/delta ring then
Daniel Defense has this one too. BCM also has
this stripped barrel for $230.