I really believe that the murder was committed by more than 1 person. It's not possible for 1 person to have controlled all the family gun or not.
There have been threads on this.
Jeremy told the police Neville had rang him saying 'Sheila had gone crazy'. He later told the police Neville may have said 'she'after the police questioned her two bullets.
Jeremy said he was asleep in bed, so either Sheila alone massacred everyone. Or one/two random burglars/psychopaths broke in and killed everyone.
I am surprised Neville had time and decided to ring Jeremy in either situation.