Play Zynga Poker for a free Mafia Wars sleeve gun
The gun being offered has a very impressive attack stat of 53 pts. Its defense is a below average 23 pts. In order to get your gun, click on the Zynga Poker advertisement within Mafia Wars and play until Level 6. This should take you just a round or two. The free gun should show up in your Mafia Wars inventory page.
In case of confusion, note that Zynga Poker is also advertising the sleeve gun within their game, but they're calling it "Ace Up the Sleeve" instead: