The MD law regulating magazine size grandfathered any existing magazines in user possession at the time the law went into effect. (Oct ?96 I think).
It is legal to own a hi cap magazine or drum. It is illegal to buy or sell one within the borders of MD. Joe Curran has extended his reach to mail order guys who are afraid to ship into the Free State.
It is completely legal (I am not a lawyer but believe this to be true) to buy one at a gun show in DE, PA, WV, VA or any other place you happen to be where they are legal. I have a couple of grandfathered drums.
BTW I am in Columbia. Are you going to the Timonium show next weekend (where 20 will be the highest cap mag)?