As Sheila had no experience of guns and had never used one in her life, coupled with the fact Jeremy Bamber obviously lied about leaving a loaded gun in the kitchen, if Sheila had really gone psychotic (which again, was impossible as she was dosed up with tranquillising Haloperidol, and Schizophrenics RARELY kill, nor have psychotic episodes when they’re medicated) but let’s suppose she had suddenly gone berserk, why didn’t she simply take a large carving knife from the kitchen, go upstairs and slit everyone’s throats as they slept — then slit her own?
Most family murders involve knives. They’re accessible, anyone can use them, you need no training, you don’t need a silencer, they're more reliable than aiming a long rifle you don’t know how to work, and quickly slitting someone’s throat and severing the jugular kills them almost immediately — and slitting your own throat too is far easier than trying to shoot yourself with a rifle that’s not much shorter than you are.
Surely someone having a psychosis attack who for some strange reason decided to kill everyone for no reason would use the easiest method?