The crime scene evidence was Sheila had loaded, re loaded, chambered & breached the rifle, fired 26 accurrate shots, fought and burnt Nevill.
Bamber had said she knew how to use the weapons & told the police where she may be hiding inside WHF.
So it may have been more realistic to just suggest she had taken one of the rifles from the gun cupboard.
He can still insinuate Sheila by mentioning a fostering conversation, Nevill's call & her condition.