You can bring back everything except the lower, and certain optics that are covered by ITAR. The upper is fine don't take any other answer. It does not contain the firing mechanism and does not have "serial numbers" on it (to the best of my knowledge).
Also mechbox != receiver, if they want to confiscate it let them, don't argue or you may find your name/info/plates on "the list". Just appeal or ask for a supervisor (or someone who actually knows what a receiver is), although it may take months if not years.
Declare everything, just say you're bringing back replacement parts for a BB gun, which is true since you can replace broken parts of your current gun with the "replacement parts" that you just got.
Just remember you may get asked questions and to answer them honestly and truthfully. It looks especially sketchy if you have rails, fibre stocks etc. You've probably got a big bad "military" auto sniper WTF machinegun-assault rifle