There are six sidearms you can unlock in Twisted Metal for the PS3.
Here are the sidearms you can unlock, and how to unlock them.
Laser Beat Dollface Event #5
Laser Pistol Complete Story Mode on Twisted difficulty
Magnum Revolver Beat Sweet Tooth Event #4
Rockets Beat Dollface Event #2
SMG Beat Sweet Tooth Event #1
Twelve Gauge Shotgun Beat Mr. Grimm Event #4
Check out my other article for the full list of unlockables in Twisted Metal for the PS3.