If you want light weight, get a vfc m4. They use a very light and sturdy metal, either aluminum diecast or magnesium diecast. They are usually in the 250-450$ range
My CA M4 is bulletproof, only thing that's ever gone wrong with it is a blown fuse, and that's just because I haven't gotten around to converting it to new connectors. They also have licensed Armalite trades which are very nice.
ISAF-C, The 144, Rushing Russians OR/Willamette Valley KВДВ