The maximum range of a sniper rifle blaster bolt is 1000m. Hence the difficulty for shooting a man at 1000m is 71-75. Also a factor is the movement rate of the target. That is why movement rules are important. However, a target can dodge if they know that they are being targeted.
Snipers are very precise and calculating individuals.
They wait for shots at their targets. Suppose a sniper with 9D blaster wants to shoot at two targets at 500m. He aims at one for a moment. He only gets the bonus of the scope and aiming roll for the 1st shot. On his next shot he loses -1D because of multiple actions and doesn't receive any bonuses for aiming.