The guy that runs Gunrunners in downtown Frederick is a really really nice guy. They had a decent amount of guns in there last time I went, but his shop is pretty small. The owner was always very helpful with anything I needed. I would still be going there if I still lived in MD.
His hours are a little funny, but its worth it to go see him. I think he does tuesday to friday 5pm to 8pm and then sat I think 9-5