There seems to be a level of incompetence in all these shootings, which suggests the person who did the shootings was far from being a crack shot like Jeremy? More likely to have been someone who was not very good with guns...
I think you need to make your mind up, one minute, depending on what theory you are presenting, you're trying to suggest that Jeremy didn't know much about firearms, then the next your are suggesting he is a 'crack' shot.
Well the truth is actually more likely to be somewhere between the two.
Then we have the wounds to the five victims, you suggest that the wounds are spread all over the place, a little inaccurate, yet on the other hand, not a single shot failed to hit one of the victims, so perhaps they could be described as more accurate than you suggest.
In fact for Sheila, who was inexperienced with firearms, to have fired 25 shots, whilst being in a psychotic rage, and somehow hitting the target with 100% accuracy, could be considered highly unlikely.
Your example of needing several shots to murder the twins and somehow trying to link it to Jeremy's ability with firearms, whether he was a 'crack' shot or not is irrelevant as these particular shots were contact shots, the number of shots fired also does not have anything to do with Jeremy's ability.
Another interpretation of the shootings, would be that shots were sadistically fired to cause pain to June and Neville, before finishing June off with an execution style shot between the eyes, perhaps the same was intended for Neville but he had a lot more fight in him than expected.
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Given enough time we could make anything fit, which is precisely why unless these speculative theories, are based on some sort of evidence, then they aren't worth a damn.