The final battle in Battleship (2012) brings the WWII battleship "Missouri" hilariously into play against the alien mothership. They wave away the fact that the boat is operational and fueled with a comment about "enough fuel for a maintenance run," but where do the shells come from? They go into battle with dozens of explosive shells, and we're told that some of these, at least, weigh 1000lbs each. Were they just being stored on a decommissioned naval vessel being used as a museum and tourist trap? Or was there some explanation within the movie that I missed?
As far as I remember, visiting the Battleship New Jersey, there was plenty of ammunition lying around -- I mean the shells were there. I think they were AP shells, in that they were generally big lumps of metal, so, there was nothing that would or could explode -- hence, why there were a large bunch of them sitting around.
In theory in the movie the shells could have been sitting around the grounds, doing nothing but waiting to be used in the display around the ship.
The only factor I would see is where did they get the gun powder?? That's the tricky bit, I see.