The shells probably hit the boxes along the sniper's nest before they hit the floor, and that's going to make a more of a "thwack." Anyway, based on my not-inconsiderable experience at gun ranges, empty shells don't really make a "ping" sound when they land on a hard surface. It's more of a skittering sound, for lack of a better term. That's because they tumble and roll when they hit the floor.
I simply selected the word "ping" to demonstrate that the shells would have made a noise when they dropped onto the floor, You can use any word you like .... ( aside from a loud crash)
I had hoped that an alert and astute researcher would have rebutted me, by saying that Norman testified that he had heard the shells hitting the floor above his head.... If that was the truth....Then why didn't he include the sound of the shells hitting the floor when he spouted the BOOM Clack ....clack....BOOM Bull stuff?