The gun belonged to David

The gun belonged to David and was kept in his room. 

Robin, at the time of the killings was living in a caravan outside the house.  There is some controversy over who did and didn't have access to the key lock.  This is only relevant if the rifle is actually locked, but what is important is that

  1. The rifle was in David's room
  2. David had control over where it would be at any given time
  3. David was familiar with it and was more capable of sorting out a misfire problem whilst in the middle of a life and death struggle with Stephen than Robin would have been.
  4. He also admitted at the first trial that he was the only one who knew where the key for the trigger lock was.