I have a fictional character in 1870 who is a marksman of superhuman ability; basically, he doesn't need a sniper rifle with a telescopic sight, he is a sniper with a telescopic sight, and can get the maximum physically possible accuracy from any weapon.
Looking at the weapons available at the time, this one seems representative: https://en.wikipedia.org/wiki/Snider%E2%80%93Enfield
Effective firing range 600 yd (550 m)
Maximum firing range 2,000 yd (1,800 m)
I understand these two specifications to be, respectively, the range at which you can hit a man-sized target with reasonable probability, and the maximum distance the bullet will travel and still retain a lethal amount of kinetic energy.
I'm trying to figure out what would be the effective range of this weapon in the hands of a superhuman marksman. It seems to me the best way to approach this is to ask, what is the limiting factor on the effective range?
Is it the accuracy of a human marksman?
Is it random environmental factors like the wind, that a superhuman marksman could conceivably compensate for?
Is it some 'random per unit, but fixed for that unit' variation in the rifle itself, that the marksman could conceivably learn to compensate for?
Is it inherent randomness in the system, impossible to compensate for, that will limit the effective range to 600 yards regardless of the shooter?