I haven't played back the recordings, but I've never seen a score in the millions by playing the game through.(posted 9612 days ago)The cheat is. At the end of the Zombie section there is a spot where you move the chracter left then right, each time a zombie comes out. Shoot each time you change directions and you kill a zombie.
If your coordination is good enough (I'm sure every here is :), you can score enough points to get an extra life. Thus playing pretty much forever.