Quincy's revelation that the bullet lifted the shelf before entering the wall, and that the shelf then lowered and hid the bullet, is the same scenario faced by Sgt. Joe Friday in The Shooting Board (1967).
Dabney Coleman (Officer Peter O'Neil) played the part as Dr. Burt Travers in Quincy M.E. season 2, episode 8 called "A Good Smack in the Mouth".