Do you think the killer underestimated the number or bullets required?
I can't believe this. He had been planning the massacre and knew a bit about guns.
Ideally he would be hoping to kill everyone with the 10/11 bullets. As suspicious people may ask if Sheila could chamber and re load.
Maybe one shot head each into Sheila and the twins, the rest into Neville and June. However Neville got downstairs after only two upstairs head shots.
After Neville's beating, more head shots were needed. Meaning a re load was needed as he had already fired bullets into June and two bullets into Neville's torso. Sheila, Nicolas and Daniel had still to be shot.
Once he had chambered and re loaded once, the damage was done. He might as well chamber and re load twice and fire lots more bullets into people to make it look 'crazy'.