NO ONE should write anything to do with firearms unless they become a certified firearms instructor to learn WTF theyre talking about.
Spent bullet on the floor? The one that went through the wall? The was most likely empty brass case, or a cartridge. What a bunch of morons.
We do not know what it hit inside the wall. It went through the mirror, the plasterboard, maybe hit a stud, and went through the plasterboard on the other side.
Then we do not know what it hit inside the apartment on the other side of the wall before it came to rest.
An expended case is unlikely because it was found in the apartment on the other side of the wall, and it was a revolver that was fired.