In a holodeck, most of the items are holographic, but according to ST: The Magazine (now sadly out of print) If a holographic item is picked up, a "prop" is replicated, unfortunatly that contradicts an early TNG episode where Data threw a book through the arch and it instantly vanished. Of course we can put it down to advances in holo-technology. This Prop is exactly the same as if you were to have replicated it yourself, so the tommy gun that Picard had picked up was real, and since the safety protocols were off, so were the bullets.