In the place where you are chased by the zombies at the very beginning of the game (after you jump over the indusrial bin) go to the place where you get the back door key and there should be a cupboard at the back of the room go there and press x you should pick up gunpowder a (do this twice) then go to the box next to the typewriter and press x it should open. Trade one gun powder for the magnum and the other for its ammo.