In the game you find DDK puzzles. There are seven altogether.
Here are the codes for them.
7. Doctor kirk
There are some areas that you will only pass through
once or twice in the entire game. Killing the dinos in these areas
are pointless. More importantly, it is a waste of ammo. Use the
weakest tranquilizer dart on them, and move on. This way, the
bullets/shells/slugs/grenades are available when you need them.
Once you get unlimited grenades (by completing the game three
times under the normal difficulty setting), you can kill those
dinos, without wasting ammo.
To unlock the weapons vault in the lounge the pin is 8159
In order to get infinite bullets for the Grenade Gun, you must complete Dino Crisis a third time, but this time you must get a different ending from the one you got the first and second times you completed the game
To unlock the Army and Battle costumes, you simply need to complete the game for the first time.
To unlock Operation Wipeout, you must complete the game under 5 hours.
In order to unlock the third and final costume, you must complete Dino Crisis a second time, but this time you must get a different ending from the one you got the first time you completed the game