If Norman described it as "BOOM-clack-clack, BOOM-clack-clack, BOOM-clack-clack", then it only means the first round had already been chambered before Norman was aware of that there was a rifle above his head, or before he was in a position to hear it.
If Norman hadn't been fantasizing about what he claimed that he had heard.....He should have heard
BOOM---CLACK--- ( bolt opening)---
PING--- ( shell hitting the floor)---
CLACK ( bolt closing )---
BOOMFIVE separate and distinct sounds......
That would have been the sounds of the first two shots.......However , all of the witnesses said that the third shot followed the second without delay....
Mr Norman was lying....He never heard any such noises.....