Press START to pause then enter one of the following codes:

Earn one continue: A, B+Left, A, B, A, B, C, A
Fill weapon to 1,000 rounds: A, B, B, B, C, A, C, C
One free guy: B, B, C, C, A, A, A, A
Plasma Shot Weapon: C, A, B, C, A, B, A, C