Note: This is only possible to get during Dolores' chapter.
In the basement, next to the storage room and the "house rules", is a cramped closet with a mirror inside. Break the mirror, enter the broken wall, and slide through to the other side. Open the door to see a large mound of large, wooden crosses. Pick up the one straight in front of you to open the path, but don't drop it. Instead, take it with you as you go through and up to the other side.
You'll have to temporarily drop it to get up and out of the hole. Make sure you pick it back up afterwards! Then go up the stairs. Once you get to the circle of candles, move to the right, and go through the open doorway.
Place the cross on the hole in the floor, then use it as a makeshift bridge. Walk across it and enter the next room. Here you'll see the pump action tactical shotgun! Pick it up! It's action time! Achievement pops!