You're falsely equating the environment Norman was in with you cycling a Carcano in a sterile setting.
You're kidding.....Norman senses would have been on high alert..... so if he had actually heard the sounds ( especially the loud, ear shattering, muzzle blast) that he claimed he had heard, he would have immediately notified his companions that a gunman was firing a rifle on the floor above them. But It's very doubtful that Jarman and Norman would have needed Norman to tell them that a rifle was being fired on the floor above their heads.....They would also have heard the loud blasts of a rifle being fired.... Can you explain why only Norman heard the sounds?
And we know he didn't do that.....In fact he never mentioned anything about hearing "shells hitting the floor"until a couple of days had passed and after a few suggestive interviews with the FBI.
IMO, Norman was a simple kid and eager to please the authorities....
