Nevill or June would have instantly reclaimed the rifle. Not rang Bamber's AM at 3.00am.
Sheila had no co ordination, balance, strength, alertness & sufferred from involuntary movements.
Nevill was taken by surprise and initially bashed over the head and knocked out beside the Aga when Sheila kicked off,and as you know June was shot in bed before she could reclaim the rifle or anything else,once she did get up and grapple with Sheila she was too badly wounded to overpower her,not that June stood much chance anyway against a psychotic Sheila.
Nevill called Bamber at 3.10 once he came to lying against the Aga.