Because a bullet that grazed Nevill and 4 casings associated with the 4 shots to the left profile of his body were found in the bedroom. The gun would have been empty after these shots and those to June were fired which explains why things then progressed to the Kitchen and there was a struggle. Either the killer ran to the kitchen to reload with Nevill chasing or Nevill ran to the kitchen to arm hiself or the like and the killer gave chase and in the kitchen the killer had to beat Nevill unconscious to be able to reload in peace then fired the 4 shots to Nevill's head.
Seven bullets fired, producing 8 bullet wounds (Ralph Bamber), all seven bullets recovered from inside his body.
Seven bullets fired, producing 7 bullet wounds (June Bamber), four bullets recovered from her body, three loose bullets recovered from the main bedroom.
One loose bullet, recovered from the main bedroom.
Two bullets fired, producing two bullet wounds (Sheila Caffell), two bullets recovered from her body.
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12 cartridge cases recovered from the main bedroom, producing 13 bullets, linked to same bedroom, do not add up, something is very wrong with your calculation:-
Lets go through the known facts, of who got shot in that bedroom according to you...
June Bamber, she got shot 7 times, which must be linked to 7 of the 12 recovered cartridge cases found in the main bedroom, leaving 5 cartridge cases to still be accounted for - three bullets recovered from Ralphs body, all supposedly fired into his body whilst he was present inside that bedroom, so this takes up another three of the five cartridge cases, leaving two cartridge cases to be accounted for. All appears well up to this point, but then things start to take a turn for the worst, because we still have 1 loose bullet recovered from the bedroom, and two shots into Sheila to deal with, producing a total of three bullets requiring three cartridge cases to be matched to, but where only two exist. Now, one of the two remaining cartridge cases must relate to the loose bullet discovered in that bedroom, and this leaves just one cartridge case against which to match to two shots into Sheila's neck, but only one of these shots can be accounted for by the solitary cartridge case remaining to be accounted for in that bedroom...
We are left with, one bullet fired into Sheila's neck, without a corresponding bullet case found in that bedroom...
The only reasonable conclusion one can arrive at, is that one of the two shots fired into Sheila's neck, was inflicted in another part of the farmhouse...
Do you not agree...