Note: The following trick requires a disassembled Atari joystick in order to activate two opposite directions at the same time.

Hold Up + Down and tap Fire. At times, you will lose your gun. The gun will remain in one place and the robots can get killed if they walk into it. This may also work by holding Left + Right when firing. And if you try to shoot after losing the gun, your character's arms will rise but no bullets will leave.