Resident Evil 4: Wii Edition

The first magnum you have access to, the Broken Butterfly, can either be purchased from the merchant, beginning at the start of chapter 3-1, or you can get it for free at the start of chapter 4-1 by backtracking to the outside castle area where the water fountain is, just before the hedge maze. Examine the door, lift Ashley through the window, then enter the room and open the center chest to find the BB.

You can also purchase the other magnum, the Killer 7, upon meeting the first merchant you encounter in chapter 5-1, about halfway through the level.

Please note that both magnums specifically require magnum ammo (black boxes), NOT handgun ammo (orange boxes).