And why bother going to the trouble of building a hiding place for the rifle, try to wipe it clean of all prints but leave the empty shells just lying there.
Because ...as another poster pointed out the spent shells were "bread crumbs".... To lead investigators into believing that the shots had been fired from that site and at the same time lure the cops away from the picket fence.
I believe that Lee Oswald was the person who threw down the "bread crumbs" because he wanted investigators who would come looking for the person who had taken a pot shot at JFK to believe that the shots had been fired from that site. And he also hid the rifle beneath that pallet of books because if he had left the rifle right there at the window it would have been found and traced to his PO box. He wanted the rifle to be found but not until he had sufficient time to get out of the U.S. and on his way to Cuba.
Lee had no part in luring the cops away from the Grassy Knoll....But the plotters who set him up most certainly wanted the cops lured away from the picket fence where one of the gunmen could have inadvertently left some evidence ..... Like a spent shell for example.