Hi, I have a brother retired Chief, trapped behind the protective iron fist of the Benevolent Democratic Peoples Republic of Maryland, who owns a pre-2012 S&W M&P M4 type AR carbine. He wants to change the upper out to something like an 18" straight profile barrel with a free float handguard, flat top. Since it was legally owned by him prior to the latest Maryland "Sandy Hook knee jerk" laws being passed, what are the legal implications associated with this project? This is not a post-2012 AR, and is not a "Heavy Barrel" variant. It does not have a flash hider however. Along with the previous questions, can he add a flash hider to his new upper? Thanks brothers! Resist!