Change the fps on a mini bb gun?

how do you lower the fps on a mini bb gun. me and my friends play tag with bb guns and when the bbs land on our skin it hurts like hell cuz we play in a small office like building. pluse we sometimes get in trouble for hitting per byers. here is the gun we use a href="http://gun-sword.stores.yahoo.net/dppaafuaumie.html" rel="nofollow"http://gun-sword.stores.yahoo.net/dppaaf…/a