I always thought that as the gun was quite new and Sheila had probably not even seen it before, it was quite convenient that Jeremy gave her a demonstration by loading it in front of her!
I also thought it a coincidence that, if she had been looking for a gun, she chose that one too! There were plenty to choose from that she may have been more familiar with. A shotgun would have got the job done, but she picked up the 2.2 and then decided it needed a silencer!
Has JB ever expressed any regret over this btw? .."If only I hadn't left that gun out..." - Most people would feel some responsibility and guilt for their part in a massacre.
I'm not certain of the time-line, but I think said gun had only been purchased the previous November. Sheila didn't visit the farm over Christmas because she was unwell -I suspect she was building up to the breakdown- so quite when she'd have been at the farm and well enough to take instructions on something which held no interest for her, is anyone's guess.
"That" gun was conveniently there, asking to be picked up by anyone in murderous rage.
For most of us, it would have played on our minds for the rest of our lives. I don't know if Jeremy has ever received counselling to try to work through his guilt, regarding it, but to my knowledge, self recrimination hasn't figured large.