Code notes:
The following codes are valid for the Japanese version of the game.
No codes exist for the North American version. 

Level select:
Press Left, Down, Down/Left, Left, Start at the player selection
screen. A hum will confirm correct code entry. 

All weapons:
Press Right, Down, Down/Right, Right, Start at the player selection
screen. A death scream will confirm correct code entry. 

Screen spin:
Briefly press L or R once, then hold the button in levels two or
four. The screen will spin rapidly. 

In-game reset:
Hold L + R + Start and press Select during game play to return to
the title screen. 

Sound test:
Rapidly tap X while quickly pressing Down, Right on the title
screen.