Seems the defence scenario is -
Sheila collected the rifle while everyone slept.
Fired 9 shots at June & Nevill in the main bedroom.
Nevill got downstairs and started ringing Bamber.
Bamber answered and Nevill said 11 words before the phone went dead. Which suggests Sheila had arrived in the kitchen.
There was then the kitchen attack. The injuries and kitchen crime scene well documentated.