Yes all A rankings or something gets you co-op on all of them. You can look it up. Also all S rankings on hard gets you unlimited ammo. That's a pain. Took me forever. Get House of the Dead if you like light gun games it's good.

I don't think it needs to be on any difficult or getting any ranking. When I was going through it, I kept getting my arse kicked, so i knocked it back to easy because all I was trying to do at the time was unlock the coop.

Like I said guys thats how you unlock unlimited ammo. All S rankings. I think you have to play through all the levels once to get coop but I think you need all A rankings or something to get the last two levels.

Co-op for the single player missions is unlocked by simply have to clear 4th survivor single player mission, difficulty doesn't matter,, that however requires getting 2 or 3 A rankings, I know you have to clear Racoon's Destruction 3 with a A, which unlocks the fourth main scenario, and you still need to beat a few of the single player missions with an A

Ok, I guess I'll have to put some more time in the game... Getting A ratings is hard with my co-op partner as she isn't a hardcore gamer and often misses the "duck" opportunities and rarely shoots out stuff. It would be nice if there were codes to unlock it all...

Sorry I missed the edit by Rol. What he said. LOL. Also, HOT 2and 3 are very good and very short so its up to you.

a way to make it easier to get an A ranking is to play through a chapter that requires an A and make file collecting your priority, having that free S in the file category makes getting A rankings a whole lot easier,

stock piling weapons doesn't hurt either 

the only missions you need A's on to unlock 2 player are

Nightmare, unlocks nightmare 2

Beginnings unlocks beginnings 2

Raccoon's Destruction 3 unlocks Death's Door

these all unlock various other chapters, simply clearing every chapter unlocks fall of umbrella, which unlocks Dark Legacy, clearing both dark legacy chapters unlocks 4th survivor, 4th survivor unlocks complete co-op