It doesn't make sense to me that none of the three adults couldn't have even fired a gun outside to attract attention. There were cottages nearby who would have heard a shotgun going off in the night outside of the farmhouse.
A chap walking his dog that night at 10pm or thereabouts, actually heard the sound of a gun being fired. He gave a statement. At that time, Nevill would have still been in the fields as he left for there after JB left for home after supper. The twins could already have been shot-----we DON'T know before Nevill got back.
The chap's statement is probably with EP along with " Taff's " notebook and other original notes etc when it was 4 murders and a suicide.