Visited the NRA Museum just this last Wednesday. They had on display as a Special Exhibit the Tommy Gun in history. It's well worth the time. They also have a display of modern flintlocks including a coule made by one of my instrutors, Jack Brooks.
About the only display I would criticize is that one on the Civil War firearm. Too many guns in too small an area so you don't get a good view. Considering the limited gallery space, this is understandable (but unacceptable). If you want to see Civil War guns, go to Gettysburg or Chickamauga National Battlefield Site.
Be careful of the NRA Museum giftshop/bookstore.