Certainly I think it's much more likely SC would do the twins first (and pull so hard on the trigger that the gun fired more than necessary - but perhaps one shot each at first and then returned later to be sure), but that JB would do Nevill first: which is why I find it odd that it is was June who most of the action seems to have been aimed at in the bedroom. I don't think Nevill was in the bedroom when June was shot, I think he was downstairs, either up and about for something farming-related or talking to SC. I read somewhere that only SC had partially-digested food in her stomach even though they had all eaten together, indicating she was up and ate something during the night and perhaps this disturbed Nevill and he went down to join her, then whilst putting the kettle on maybe he heard shots above his head from the boys room, phoned for help, dashed upstairs whilst June was being shot on the bed, was beaten back with shots to the face and arm fired from within the bedroom, ran back downstairs and collapsed in his chair, where the assailant beat him with the gun which was reloaded for more finishing shots to all the victims.
I think the marks to the arm most look like he was hit with the rifle barrel with the integral sight making the rounded bruises: and that strikes me as quite a feminine way of hitting someone with a rifle (in those days perhaps?). Having read up on and posted links to the surprising ability of the brain to keep functioning despite being shot, allowing people whose front temperature lobes are damaged to keep running for up to 20 secs, I'd say NB was only able to react instinctively and turn to protect his injured left shoulder and raise his right arm over his head to receive the blows. I don't think he was able to struggle, he was already too badly injured. It would be obvious he could not call for help and he was on the other side of the kitchen from the phone: whoever beat him was either very angry or panicking he wouldn't die. I would have thought at this point Bamber would know he stood between the phone and Nevill, he had the bullets in a box beside him and he would have calmly reloaded without the panick. But then I think he would have known the likelihood of a .22 killing a man as big as Nevill and shot him in the head repeatedly as the first victim. Hitting with a 'stick' is more the sort of aggression I'd expect from a feminine woman not used to fighting. And I believe she had previously set about SC hitting him as well? (Do correct me if I got that bit wrong!)
