The most plausible explanation for the silencer is -
Bamber used it for a silent massacre attempt.
The rifle scratched the aga during the huge kitchen attack.
The silencer aquired a scratched during the huge kitchen attack.
A hair attached to the silencer when Nevill was being shot in the head.
Sheila's blood went into the silencer when she received 1/2 contact shots in an area of high blood flow.
The silencer was taken off after shooting Sheila as she could not shoot herself with it on.
The silencer was put away in it's usual place away from the crime scene when Bamber went downstairs.
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Understand why Bamber has focused on the silencer since the mid 90's as the relatives found it.