You can fire gas rounds without depleting any ammunition if you use the gas launcher during "run" mode.
In your first save file get all 3 characters Barry, Leon, Lucia then get to the nearest save point and start a new game and all the characters will be there (this also work if you want Barry and Leon only)
To get the glitch to work, go through the game until you get off the sub and have to fight the B.O.W. in order to get back onto the deck. He should be around some elevators next to the middle entrance to the deck. Fight him, and beat him. You will proceed outside, and have to fight him again immediately. This time, LOSE. Let him kill you. Now, select the option to continue your previous game. You will start right in front of the door to the deck, after you defeated the B.O.W. for the first time. BUT, the B.O.W. will be back! you'll have to fight him again, but this time, all of the ammo you used to kill him the first time will be gone.
Use this code to get infinite health 923091d4.
Use this code to get infinite ammo 910ac1c3.
Codes are for gameshark only.