I have to correct myself this soay island
http://www.theislandreview.com/8-shape-shark-isle/

is another than the home of the forgotten acient sheep race
https://en.wikipedia.org/wiki/Soay,_St_Kilda

there exist a book by Maxwell about the time when he and some other man were hunting basking sharks

will take a look into this if he gives more informations about the use of this ancient greener gun