Personally I would enter the main bedroom first with a fully loaded rifle. Neville was potentionally the biggest threat. Neville was shot 4 times upstairs. June shot seven times, on two separate occasions. So it seems Jeremy entered the main bedroom fully loaded.
There is a chance Jeremy shot the twins. Then went downstairs to reload. That is risky. If Neville & June confront an out of bullets Jeremy upstairs, it could be curtains.
We know the twins were still asleep when they were murdered.
We know Nevill was shot in the bedroom and then in the kitchen.
We know that June was awake either before but certainly after the first shot at her.
My theory is that everyone in the house was asleep, JB entered the twins room with a fully loaded magazine in the rifle (with sound moderator on) and he also carried the box of ammo (or extra loose rounds) on his person. He quickly murdered the young children in cold blood. He then reloaded.
Next he entered the main bedroom, Nevill and June may now be awake thinking that they had heard a noise. JB shoots at Nevill and June, hitting them both a number of times. Nevill forces his way past JB on to the landing (where he leaves blood smears on the wall), down the stairs and in to the kitchen (leaving more blood smears outside the kitchen door. JB now without any ammo in the magazine chases after Nevill and attacks him in the kitchen, striking him with the rifle which causes wounds to Nevills arms and eventually knocks him out after being struck so hard with the butt of the rifle that the butt splinters and breaks. JB reloads and shoots Nevill four times in the head.
During this time Sheila has woken, entered her parents room, finding June slumped on the floor but still alive, she goes to check on the twins, June gets to her feet and staggers around the room and then heads towards the landing...........
JB makes his way back upstairs and meets June in the doorway, he shoots her and kills her.
Sheila appears back in the door way leading to the twins room, she is in shock. JB threatens and coerces her on to the ground and shoots her. He then thinks that he might be able to make it look like a suicide, he stages the scene. He realises that with the sound moderator attached, it might not be possible for her to have reached the trigger with her hands, he removes the moderator and repositions the rifle, but Sheila makes a noise, she's not dead, luckily there is one round left, he shoots her a second time which kills her instantly. He throws the bible on to the floor for effect, heads downstairs, hides the moderator in the gun cupboard, takes the phone in the kitchen off the hook and heads back to his house.
He cleans himself up, calls the police and Julie and the rest is history.