Upon entering the Museum of American History, make your way to the Lower Levels, located on the left as you enter the big room with the giant prehistoric animals. Once in there, walk up the left staircase and go into the room filled with empty displays and enter the door to the offices near the back left corner of the room. Make your way through the offices until you come to a large, open room with a turret on the ceiling. Walk up the staircase against a wall in the front of the room. When you reach the top, you should see a giant hole in the wall. Once you reach the top go through the giant hole, and in farthest cubicle on the left you should find the rifle