Originally posted by: Mikey
Imagine some evil scientists have taken Jessica Alba hostage. They want to turn her into a super zombie. You hear her cry for help, "SOMEBODY SAVE ME! THE ONE WHO DOES GETS POON FROM ME!" You have 48 hours to rescue her before she starts eating you out (hehehe). Everyone else is dead, so no help is on the way. All you have is a pair of jeans, a pair of converse, a black T, a kevlar vest, and a swiss army knife. All help is dead, and the city is under strict quarantine. 
What single firearm will you choose? Be realistic here. No fanwanking allowed. As for ammo (if any), it will be scattered all over the city (from dead soldiers). 
Personally, mine would probably be the P90. Strong, fast, and plenty of stopping power with the armor piercing rounds.