Thus the adage: "ALL guns are loaded."
I would bet that the owner of this particular rifle had removed the magazine and/or emptied all of the rounds out of the feed tube without checking the chamber.
That said, the gun did not jump off of the wall all by itself, nor did it pull its own trigger. "Negligent discharge of a firearm" is just that.
The fault is with the perp, not the owner and not the firearm.
I guess I better do something about the loaded Winchester Model 1887 on the pegs over the doorway...