Sheila would have put the silencer on the rifle when she went downstairs.
She could have also loaded a seperate magazine with 8 bullets.
Then gone upstairs with the rifle/silencer and magazine.
Once she had emptied the rifle on June & Nevill - 9 bullets, she went into the twins room to load the magazine and shoot them 8 times.
Sheila being in the twins room gave Nevill the chance to go downstairs and start phoning Jeremy.