Some interesting posts, I'm (slowly) in the process of building an AR15 pistol in 7.62X39. It's good to know I don't have to worry so much about sights and the weight from them. Some of my observations:
The 7.62X39 barrel might actually help reduce the weight, the outside diameter of some of the 7.62X39 barrels is the same as 5.56X45 barrels, which means a lighter weight for the same length barrel. Flutting the barrel might help as well.
I'm using a Tannery 80% Ceramic Matric Metal (CMM) receiver, the same weight as the standard aluminum receiver but 40% stronger. This will allow me to reduce weight but keep the same strength as the standard receiver.
I'm planning on machining out the inside of the receiver as much as possible while keeping the standard AR receiver look. Rather than drill lightning holes, such as the OA98, I will thin the material where (I think) I can and make a webbing structure. I figure I;ll drill as many little holes in things, such as trugger, mag release, etc. as I can.
My upper receiver is (unfortunately) the Hesse/Vulcan composite receiver, I understand they aren't as accurate, due to poor tolerances, primarily the loose fit of the bolt in these receivers. That might be taken care of with a sleeve....