If Sheila did not take her own life...
Perhaps the killer shot her once - the non-instant but eventually fatal wound - went to place the gun, realised the mistake regarding arm length, removed the silencer and shot her again, full contact so the length would fit.
Perhaps they were then distracted - Neville not quite as incapacitated as first thought or even June - and had to deal with that.
Perhaps the silencer was taken then put back.
Perhaps, perhaps, perhaps...