Good god. I know where he got this god forsaken idea. He's even said he's a heavy gamer (which I know I am, too) and he got this idea of his from Assassin's Creed II.
In the game, Leonardo da Vinci makes for you a small "gun" that fires a small musket, essentially (it's roughly the size of a 9mm).
Please, for the love of god, do not encourage him whatsoever.