-MISSABLE-Note:
Only obtainable with the playable character
Jill Valentine!
This is right after you entered the metal gate leading to a underground tunnel located beneath the main stairs of the main hall in the Spencer Mansion, which needs the
two Stone & Metal Objects.
There is a typewriter and an item box (down the stairs). Make a save-file if you want to keep
Barry's 44 Magnum (more powerful than the general Magnum Revolver). You can get this achievement and simply reload, choosing the other option to obtain it. Although this will negate the following achievement:
Continue the down the tunnel path and trigger a cutscene. When it ends you get a question:
"Give the gun back to Barry?"Answer
YES and you will unlock this achievement! Careful though, this triggers the actual bossfight with Lisa Trevor.
Hope this helps!