These codes give you the gun with infinite ammo (not clip ammo!). ONLY USE the codes for weapons you know you'll use for example, only turn on codes for guns in your favorite weapon set. The game WILL freeze if you turn on a code for a weapon that's not in the level. They work in story mode for players 1 and 2 as well BUT you must know what weapons are available to you so you know what ones to turn on. GameFAQs should have available lists. For grenades you must press down on the D-pad to use them initially (change your grenade). It IS OK to have the Pistol 9mm in your weapons set and use the Pistol 9mm x2 code. The only problem with that is when you switch weapons, there will be no icon loaded for your Pistol 9mm x2. Using 2 codes for similar guns, like the K-SMG (& RPG) and K-SMG x2 (& RPG) codes, at the same time WILL NOT cause the game to crash. Finally, giving 2 or more different codes the same name in the Action Replay causes them to replace each other. You don't want that to happen. ;p I recommend putting a P1, P2, P3, or P4 at the start or end of each code title to avoid that.