There are many things which come into the equation at long distance.
Starting with the easy bits, gun fit as mention above, trigger control, reproducible hold off a stable platform, consistant mv, range finding. The not so easy bits are optimising the rifle power to the best pellet for best accuracy and maintaining it. The very difficult bit is reading the wind, not just at the muzzle. Sorted fac air will be deadly at 75yds in good conditions, but believe me though 100yds is only another 25yds further on it is a quantum leap to cross consistantly

neil