When the police entered the front room of the Bain house they found Robin Bain's dead body beside a coffee table. Attached to this front room was a small alcove divided by some green curtains. The gap between these curtains was about 4 inches. Inside the alcove was a computer, and by the computer chair was found an empty 22 caliber shell casing, the police said if someone fired the rifle from the cover of the curtains the shell casing would have ejected to the right and landed where it was actually found, this implicates David because the chances of the rifle being fired in the front room and the shell ejecting and flying through the 4 inch gap in the curtains to land by the chair is almost nil. The evidence here strongly supports murder rather than suicide.
A demonstration of how the shell ejects to the right of the person shooting can be seen HERE watch closely, this is with the actual murder weapon.