Because it's easier to believe that Sheila went into one of her rages and grabbed a convenient gun with a loaded magazine right next to it, than it is to believe that she would go hunting in cupboards for weapons, and then know which ammunition goes with it and where to find it whilst in that state.
You overlook something of importance and significance...
Police believed a shotgun was fired inside whf during the incident, which caused them to seize a 12 bore shotgun and an empty 12 bore shotgun cartridge, and to fingerprint the same...