It is interesting how that evolved. I have read some books about US Marine and Army snipers. Two of the books listed out shot counts. I can't find Hathcocks book right now; I think it had a more complete list going back to the US Revolution. It list out how there were two or three shots per casualty in the Revolution, then more in the war of 1812, more in the Civil war, especially as Gatling and Hotchkis guns came on line.
"Inside the Crosshairs; Snipers in Vietnam" by Micheal Lanning, lists WWII as 25,00 rounds per kill, Korea was 50,000. Vietnam was up to 200,000 with all the automatic weaponry used.
Marine and Army sniper, 1.3-1.7 rounds per kill.
GnySgt Hathcock held the record for a long time; 2250 yards using a 50 caL machine gun. Special mount was made so he could use his 10x Unertl scope on it...