Get the Wind Crest
In order to get the Magnum, you must first get the wind crest by solving the Bee Specimen puzzle. (See How to solve the bee puzzle – Resident Evil HD remastered)
Get the stone and metal object
In order to get the magnum, you need to get the stone and metal object. You can get the stone and metal object by getting all four death masks. (See How to get the second stone and metal object – Resident Evil remastered)
Place the stone and metal object in place
Go to the eastern wing of the mansion until you reach the northern most Corridor that leads to a blocked door. Place the Stone and Metal object beside the door and you will be able to unlock it.
Solve the guard dog statue puzzle
Walk outside the mansion until you reach the area that has statues of guard dogs. Set the red arrow to the west and the blue arrow to the north in order to unlock that door.
Place the wind crest
Go through the guard dog door until you reach a square area on the map in the middle of a corridor. Go to that square area and you will find a place to insert the wind crest.
Grab the 3 more crests
When you insert the wind crest, the rock will move and you will get 3 more different crests.
Examine the new crests
Examine all the new crests by checking their backs and pressing on the buttons you will find on their backs. The 3 crests can now be inserted in their place.
Insert the new crests in place
Move to the next grave, which is right next to where you placed the wind crest, and place the 3 crests there. The rock will move and you will get the Magnum.