Begin game play in the "Sin" level in original mode with over 70,000 points. Start aiming for the lower right corner just before entering the elevator. There is a bullet power-up or a death bullet power-up at that location. The bullets kill any zombie with one hit anywhere on the body.
In original mode, after you go up the elevator and pass the first two doors, kill the zombie that has the laser swords. Just when you are about to turn to the next door, there will be a crate. Shoot it and inside will either be a Custom Air Gun or a Chamber Infinite. Also, directly next to the door will be another crate with an extra life inside.