I think you have too many variables to know what the weight of the lower is.
What stock are you going to use, collapsible, and what one
Fixed stock and standard military, target or adjustable.
Figure this as a base the weights are about
AR15 A1 = 7.5 pound (20" triangle fore stock, fixed butt stock and standard grip)
AR 15 A1 Carbine = 6.8 pounds (12" narrel with 4" flash hider, collapsible stock)
AR15 A2 rifle = 8.4 pounds (
AR 15 A2 H Bar = 9 pounds ( same as above with heavier barrel, free floated fore end and target sights)
AR 15 M4 = 7 pounds (16" barrel collapsibel stock no carry handle)
AR frankenstien = what ever you want it to be From untra heavy Varmint to skeleton carbine with a short barrel and a huge bird cage flash hider.
Personally I like the heavier rifle mine is an A2 - Rifle. with the combination of the muzzle break and the weight recoil comes strait back for fast follow up shots.
Not sure what what you want or what the lower really matters the spring kits weight about 2 pounds on shipping so a pound of metal and 1/4 pound of grip.