For over 30 years and while writing 2 books Colin Caffell has been under the assumption that Sheila could not be responsible for the murders because she didn’t know one end of a rifle from another and Sheila would have to be a marksmen
For over 30 years that is what he has believed
My Sheila scenario has her running up the main stairs, with Nevill ending the call to Jeremy and chasing after her. She turns and fires at Nevill, projecting cases on to the main landing and goes after him back to the kitchen, continuing to fire. Nevill is trying to draw Sheila away from upstairs where June and the twins are. Nevill tries to reason with her and they struggle for the rifle, probably in the foyer, with Sheila pushing Nevill back into the kitchen. Nevill is already gravely injured, so ultimately Sheila prevails and kills him. She then re-loads maybe twice more to kill June and the twins. June might well have descended the stairs to try and establish what was going on and Sheila might have chased her back up to the master bedroom. She probably shot June in two fusillades, allowing June to crawl around the bedroom in between. She then washes and changes and kills herself.
To do this, she would have needed to be familiar enough with the rifle to know how to operate it. That's all.
We know Sheila went on shooting trips and on at least one occasion she used a shot gun. She wanted to have a go and fired it in the air.
Where does Colin say that it would have required a marksman? I don't understand that as this took place indoors, virtually all the shots were at close or close-ish range, and bullets went astray.
Also, I think you mentioned before that Colin has claimed he was trying to get back with Sheila. This ties in with what I had understood too, except that my understanding of it is that it was Sheila who was trying to get back with him and Colin told her emphatically the marriage would not be resurrected. She spoke to him before they got to the farm. I had thought the conversation was on the way there, but it looks like Colin makes no mention of such. It could be that it was before the day of the drive out there, maybe at the party that Jeremy also attended. The significance of it would be obvious.