Ammo required was nothing more than a ping pong ball.
Not only did we (and still do, upon occasion) enjoy playing with these guns, the nephews and brother-in-laws have also gotten into the act.
P.S. - I just did a search to check on these toy guns (K-Gun) and the first thing that came up was an e-bay site (three days left to bid) with a current bid of $45 for a single gun. Hmm.