Federal law says you can transport under FOPA passing through Maryland. Maryland law says you cannot import a "prohibited" assault long gun. Since I am starting in Maryland with grandfathered rifles, I am not too worried. I would most assuredly encourage people from out of state not to fly through BWI, however. If you are driving, like I said, Fed law is on your side...but don't get pulled over.
And stow your carry before you hit the state line.