Mild spoilers for Dolores’ chapter
Now, while I’m not entirely sure the ramifications of what allows this trophy to be accessible I personally came across it in the basement of Dolores’ chapter. Heading to mirror two in the closet, if you turn right there is the room with the stairs heading down with the tarp and another mirror ahead. Just to the right of THAT mirror into the hallway you should get an interact message which will say
“There is fresh paint on the wall, do you want to break the wall” hit yes, after a short cutscene a passage will appear leading down further. Open the door and the revolver will be sitting on the table
Now suspicion says that this is always there and I just happened to miss it before