To unlock cheats you can:
- Unlimited Ammo - collect all golden gun parts
- Unlimited Bullet Time - Get Gold Medals on every score attack level
- One Hit Kill - Finish the game on Hard Core
- Bullet Cam On Every Kill - Find Every Clue
- Unlimited Pain Killers - Finish the Game on Hard
So far the only way I have seen to use these cheats is in replay mode and I don't think it saves your last checkpoint. You have to pick your last checkpoint and then check the cheats you want and play.
I have been trying to Find Out if there is a way to start a new game or have your checkpoints saved with cheats, but have not found any answers to date.Hope that Helps!