Schit, I forgot about that one - I used that upper to build a pinned 14.5" M4 clone for my kid. What a great shooting rifle.
I can say this - my Faxon 16" pencil heat UP when you get on it. It gets hot. But it cools fast. That BCM pencil has been used many, many times on an Appleseed qualification, and it heats up, but doesn't lose it's accuracy. That's a pretty good, realistic pace of fire, too. It's not stupid-in-the-sandpit mag dumps, just realistic firing pace at multiple targets. The lightweight hasn't been used in an Appleseed yet, but it will go through a few.
OP, you wanna build a light rifle. That's what I'm taking from this thread. You will only make a heavy (or, heavier) rifle if you use a fluted barrel.