I think that Bamber intended to shoot both Ralph and June in their bed but I wonder if, just as he overestimated the power of the gun, he also underestimated how quickly Ralph would wake and react? Maybe Ralph leapt up (or even rolled and fell out of bed) and tried to rush him before Bamber could get a bead on him?
Certainly, which is why I find it odd that all the casings are over between June's body and the wardrobe: what's Nevill doing over there? If I was Bamber I wouldn't have cycled over in the dark, climbed through the window and fumbled around in the dark: I'd have said "Good night every body!" whilst they were at the dining table and then snuck upstairs and hid in that little dressing room between Nevill and the boys: then waited until I could hear Nevill snoring before sliding round the door and slugging several shots in his head point-blank. And I'd have made sure the yap dog accidentally ate slug pellets the day before (I can credit my father-in-law with that plot line - twice!

) so it didn't give the game away. (For that matter the whole idea he could enter the house and the dog wouldn't alert everyone sits ill given the racket it kicked up for hours afterwards)
He would shoot Nevill first. If he could shoot a sleeping child sucking their thumb he'd have had no problem shooting a child running around screaming, and the children didn't have to be anywhere in particular at all for the purposes of the story that Sheila had gone berserk.
So what's Nevill doing over by the wardrobe? If the shooter was over by the door and Nevill had thrown himself over June and then slid between the bed and the wardrobe he would have been shot in the back and right shoulder, so he can't have done that. Shots to left shoulder/arm and upper chest suggest to me the murderer was aiming at a moving target, possibly the head, but Nevill had his left arm up to defend himself: this is the action I would expect him to take if he ran into the room from the landing over to June. At this point the murderer would not be able to exit to the landing because of Nevill approaching, so would have moved back into the room nearer to Nevill's side of the bed: the bed being in between them would account for Nevill not rushing at the assailant, and the chair by the door would be next to Nevill's
left hand, hence he would not have been able to grab it as a weapon. If Nevill had been coming round from his side of the bed to the attacker by the doorway, the attacker could have retreated to the landing, or Nevill would have had to say 'excuse me please!' to squeeze round to be between June and the wardrobe and the attacker would have had no problem getting a close headshot - and the chair would have been immediately near Nevill's right hand whilst his left arm was defending his head and he would have had no problem swinging it into the gun and then the life-or-death struggle would have occurred there, not down in the kitchen.
So I really don't think Nevill was in bed, he must have entered the room and retreated. Which messes with the idea he was fast asleep so didn't hear Bamber climb through a window downstairs - which adds weight to the theory it was Sheila.