I agree with Holly on this thread up to a point.
It's not impossible for Sheila to have the know how to have loaded the gun, after all, Jeremy gave her a conveniently recent demonstration while she was at the kitchen table and growing up on the farm it would seem likely she had at least a passing familiarity with guns, even if there is no evidence of her having any recent experience or interest.
However, until the magic scientists can prove otherwise, the silencer is still a part of this case and Sheila must have gone looking for it in order to attach it. She didn't just pick up the gun and put the cartridge in and go crazy - there was another step.
Sheila must have also had the focus to travel up and downstairs to reload to fire so many shots during a sustained period of delusional violence which she was not only medicated for but such a period is completely at odds with Freddies descriptions of her having a breakdown and her psychiatrists assessment.
She must have had the presence of mind to want to make sure they were dead by reloading so many times but didn't have the presence of mind and knowledge of guns to go and fetch a shotgun to get the job done once she realised the 2.2 was not working. Odd too that she decided to try and take her own life with the 2.2 despite having just proved to herself it was ineffective at killing people with one shot.
Likewise, the injuries to Nevill do not prove on their own that only Bamber could have inflicted them but it's reasonable to concede that a 6" man would be better suited to the task. The fact is that there is not a speck of evidence that Sheila was ever in the kitchen or loading and firing the gun. There were no abrasions or bruises on her hands from wielding the gun like a club as she must have done to cause those injuries, no shards of glass in her hair from the lampshade, sugar or cuts on her feet, no broken nails,nothing.
It would be more of a puzzler if her brother hadn't confessed to his girlfriend and made up a ludicrous story about his father phoning him though.