Yep.
Last time I was in the DC area, I stopped by. Left an hour later with the wallet considerably lighter. The depth and breadth of the book titles on display is simply mind-boggling. And the staff was knowledgeable about the subject matter. A good time.
However, it wasn't nearly as good time as the NRA's National Firearms Museum 20 feet away.

The NRA Store also offers a mail-order service.