To unlock the Infinite Rocket Launcher and the Gatling Gun (with infinite ammo), beat the game in under 2'30'00 with an A ranking and they will both appear in your item box when you start a new game and reach the R.P.D.
Their inclusion is basically a reward for your efforts, so that you can play through the game with the ability to destroy every undead zombie and creature you meet. The Infinite Rocket Launcher is especially fun.
On any scenario on the Normal difficulty, get to the RPD precinct without picking up any items. Before entering the RPD, head down the nearby stairs in the entrance courtyard and you will come across a zombie that was once Brad Vickers, the S.T.A.R.S. Alpha Team helicopter pilot from the first Resident Evil game (if you wish to know the events that led to his transformation into a zombie, play Resident Evil 3: Nemesis).
Brad has more stamina than a regular zombie and won't go down easily. However, keep firing at him and he'll eventually fall. Once you have done so, examine his body and you'll pick up a special key.
Once inside the RPD, head to the Darkroom once you have opened the hallway doors with the card key that Marvin Branagh gave you and examine the lockers. Use the special key on the lockers and you will be able to change your outfit. Leon gets a different police uniform and a hippie-like costume to choose from. Claire gets a cowgirl outfit and a new gun.
You can change your uniform to default any time you like.
Before you enter the Umbrella Laboratory, take the W. Box Key from the Vacant Factory. Inside the Umbrella Laboratory, head to the room opposite the one where the Giant Moth resides. Once inside this room, head to the lockers on your character's immediate left and open the fourth one using the W. Box Key.
Leon will find Magnum Parts inside. Claire will find Flame Rounds inside.
Unlock Infinite Ammo: At your 'Item' screen press the following Up, Up, Down, Down, Left, Right, Left, Right, Aim. This can be entered anytime and ALL weapons you carry and any future weapons you acquire will have infinite ammo.
Unlock Hard Mode: Complete any scenarios in any mode to unlock Hard mode. There is a Hard mode in Original and Arrange Game.
Open the invertory and enter: UP UP DOWN DOWN LEFT RIGHT LEFT RIGHT AIM. If the code is correct, your ammo number will turn to an Infinite sign immediatly. (It's perfectly works) Of course it can be turned off, by re-typing the code. Have Fun