N503AS is taken. It's currently assigned to a Ea Vision EA J100 registered in White Hall, AR.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
N503AS has been worn before. Past registrations from FAA deregistration records:
Tails one nudge away from N503AS that aren't assigned or reserved: