@Armchair Detective
I did not say that Essex Police accidentally picked up four spent cartridges. You can't accidentally pick something up with your hands. But there would have been a lot of police activity in the main bedroom and officers would have moved from the kitchen to the main bedroom, each a focal point, and cartridges could have got stuck in boot treads and been kicked around. You also have to remember that the trajectory of cartridges is not linear; once ejected, they can bounce anywhere in the vicinity. If the killer is shooting from the landing or stairs downwards, cartridges could bounce back on to the landing and then end up in the master bedroom, as that's where the main police footfall is.
I don't accept that you can just create any scenario and interpret the evidence however you like. I find the suggestion rather condescending, since it's not what I am doing. In any event, this is a discussion forum. I don't mind you and Adam coming up with your own scenarios and commenting on, critiquing and criticising mine. I do mind being trolled. It's annoying.
Anybody who accepts Adam's scenario has to resolve certain problems. Why is June shot first? How does Nevill escape from the main bedroom? How is it that Nevill is shot in bed but leaves no blood in bed? How is it that blood is absent from the carpet of the upper main stairway, yet Nevill must have been bleeding and putting his hand to wounds? What about the stairway walls? The bannister? Did Nevill deliberately not take hold of the bannister or touch the stairway walls so as not to make a mess? Why doesn't Jeremy catch up with him?