The silencer is a large part of the evidence.
How does the rifle burn marks without silencer (if that's what they are), indicate that Sheila was shot without the silencer? 
Surely the burn marks could have been caused after the silencer was removed? Or am I missing something?
Knowing sweet f.a. about guns,I think I'm learned enough to state that hot metal does burn,but far more effectively ( sorry to sound flippant ) WITHOUT an added extension,i.e. the silencer. After having fired a few rounds,I certainly wouldn't like to grasp the end of a rifle.
Marks made with a silencer attached leaves " halos' ",whereas,without,it's a direct burn without the edges.
My theory,for the hundredth time,was that Sheila had tried the silencer downstairs in the kitchen,and found it too cumbersome to use. She wouldn't have been that aware that it wouldn't have made much difference to the sound,though at some stage,she wouldn't possibly have cared either.