You obviously don't yet understand how the rifle ejected the spent cartridges as explained already by Mystery. To achieve cartridge distribution as depicted in your plan the gunman had to be standing just inside the bedroom facing Nevill who had just got out of bed. It can be clearly observed that the seven spent cartridges associated with the shooting of June all ended up on the rhs of the bed.
For your scenario to work all the spent cartridges would have had to have ended up near the chair shown at the bottom of the depiction.
Yes Myster (no Mystery) kindly explained that spent cartridges eject from the RHS of the rifle. Prior to this I stated that I am no ballistics expert. I don't believe you or Myster are either?
It is not my plan. I believe it was a plan drawn up by the ballistics expert Malcolm Fletcher? As far as I can see he has marked out a shaded area where shots could be fired from. Whether this could be narrowed down further for each individual shot based on bullet wounds, spent cartridges and any other data I have no idea. It appears to me he is saying any of the gunshots could have been fired within the shaded area? The plan attributes cartridges 13 and 14 to June and yet these cartridges appear to be situated outside the bedroom/landing/stairs. As far as I am aware NB was the only victim shot on the landing/stairs.
Your scenario seems to be based on the bedroom door being locked with JB knocking on the door and NB leaving his bed (opposite side of main door) to answer it. JB then fired the facial shots at NB, said to have been fired at a distance of no more than a few inches, as he opened the door then the spent cartridges would eject to the right and land on the landing or the side of the open door. There are no cartriges at the side of the open door. And the cartridges on the landing seem to be attributed to other wounds eg June or NB's shoulder/arm shots.
I will need to look at the layout again of WHF as I'm trying to imagine JB standing outside the main bedroom with rifle and silencer and wondering if there was space for him to stand directly opposite or whether he would have needed to stand slightly to one side?
According to my farmer friends when a shot is fired with a semi-auto rifle cartridges can land within a few feet and don't necessarily simply fall straight to the ground.
My scenario of SC firing shots at June in bed while standing about a third way up from the bottom of the bed June's side and then swinging round to shoot NB as he came up the stairs from the kitchen to shoot him as he stood in the doorway appears entirely plausible based on the diagram and cartridge location.