John Buchan was writing in the 1920’s so It would have been shot with open sights from either a Rigby 275, a Lee Speed 303 or a Mannlicher. Admittedly it is a long time since I read the book.
The old way I was told by a stalker who had started his career with open sighted rifles was to come up the foreleg and then back a touch. They would soft nosed bullets which expanded well so behind the shoulder was where the shot would have been placed. If you recall to days of using such bullet runners were normal / expected. And there wasn’t in those as far as I know the “Best Practice Guide“ which of course all deer must read so that they understand what is expected of them when they are hit.
I don’t know if John Buchan was a hunter, in the way that Wilbur Smith certainly was, so totally understood the whole essence, feel and practicalities of hunting. Wilbur Smith certainly understood the differences between cartridges and feel of a fine or rifle compared to that of a military rifle.
Certainly judging by his Wiki entry John Buchan certainly grew up in Scotland and would have mixed with those who participated in stalking, grouse shooting and fishing - and certainly adsorbed enough to understand what went on. He was a civil servant, in intelligence and Governor General of Canada.
en.wikipedia.org
The “Return of John Macnab” by Andrew Greig is another very fine book if you haven’t read it.
Edit. (And forgive for going off thread) I have been doing the length of the M74, M6, M40 quite regularly. The first few episodes the “The Wilbur Smith Show podcast” are quite a good listen. In his later years WS still had many stories to tell but age had caught up with him so he partnered with a number of younger writers - WS sketched the story but the other writers filled them in. Two of them discuss at length “When the Lion Feeds” and other early novels. Have read most of his books many times these podcasts certainly shorten the journey.