Ann Eaton testified that Jeremy asked her and her husband about buying a five bullet shot gun from them. This was just before the massacre.
The advantages of this is one bullet results in certain death. Whether in the head or torso. So no re loads needed if each bullet hits it target.
It is much more realistic that Neville would be worried about Sheila with a shot gun.
Jeremy would have to justify why his shot gun was at WHF. That would not be hard. As he worked there & it was his family home.
The gun would carry less bullets, so a potential re load if any bullets missed. But he would have time to re load if Neville was out of the way. However if no re loads were needed, that would make it more likely Sheila was the killer. People have always found it hard believing Sheila could re load.
A shot gun is loud. And I am not sure a silencer can fit on one. But if Jeremy is quick and clinical, then people would be dead before fully waking up.
Using the shot gun means there are no rabbit stories. But that has never been believable.
I suspect Jeremy got too short notice about Sheila's visit. So had no time to purchase the gun. Using the rifle instead. Or he may have simply preferred the rifle, preferring the extra bullets, overestimating it's power & having thought up the rabbit story.
Would other people use a shot gun or rifle ?