Save your game after completing the story to make a New Game Plus. Instead of selecting "New Game," select "Load" to start a New Game Plus. Leon or Ada (depending on which game you're loading) keeps all their items, ammunition, weapons, and upgrades in their Attache Case.
The Mercenaries mini-game is unlocked when you beat the game once.
Beat the game once and these options will appear as options on the main menu.
Beat Separate Ways to unlock Leon's two Special alternate Outfits.
Beat Separate Ways to unlock Ashley's two Special alternate Outfits.
All weapon unlocks can only be used in a New Game +. This means you CANNOT unlock the Infinite Rocket Launcher in normal and use it in Professional until you've cleared the game at least once.
After you beat the main game, this weapon is offered for sale by all the merchants you encounter. The Matilda is a burst shot handgun that will use up some spare ammo if you find yourself with too much. It is Leon's old Heckler & Koch VP70 from Resident Evil 2.
After you beat the game, this weapon is offered for sale by all the merchants you encounter for 1 million pesetas. With it, you can kill almost any enemy in the game, including Leon and Ashley.
After you beat the main game on professional difficulty, this weapon is offered for sale by all the merchants you encounter and it cost a mere 0 pesetas. It is a special laser that when fully charged destroys the Plagas parasite in enemies ... if they have one. With this weapon you can also fire a "quickshot", as I call it, that will act like a flashbang and will stun all Ganados within range.
Beat Separate Ways to unlock the Thompson Submachine Gun (AKAChicago Typewriter) in the main game. Beat Assignment Ada to unlock the Chicago Typewriter in Separate Ways.
Get five stars on all Mercenaries - Stages with all characters to unlock this massive damage Magnum in the main game.
The Striker also has the ability to slightly break the game with regards to Leon's movement speed while in-game. This glitch is a rather simple one to activate however.
Step 1) Equip the Striker
Step 2) Aim the Striker but as soon as you hit the aim button open your inventory BEFORE the laser appears
Step 3) Equip another item in your inventory. Preferrably a grenade or an egg for maximum benefit.
Step 4) Leave exit your inventory and you should now be able to sprint at approximately 1.5 times the normal sprinting speed.
Note that several things can cancel this glitch. These things include taking damage, quick-time events, cutscenes, opening doors, vaulting over objects, climbing up laters, and jumping through windows.