Taking the first shot from a sniper rifle with a clean barrel. Extremely difficult, you have to anticipate where the shot will go. only after the first shot you can adjust your dope
(Data On Previous Engagement). also after a first shot is fired the inside of the barrel will set on carbon residue that will change the flightpath of the bulet.
jim started the ambush with a cold bore shot from 1'500 yards away. one shot one kill.