A one bullet-one target is rarely possible. A sniper is different and designated marksman is different. Snipers will try to be located as far as possible and the distance makes accurate shooting very difficult. That's why the target must be carefully selected. At times the sniper waits in his "hide-out" for hours/days and at other times the sniper has to "stalk" his target like a jungle predator stalks his prey for days.
Many people think sniping is very easy, lie on the ground-aim-shoot. I used to think like that as well.
It is the most difficult job in the infantry. You have to remain in a constant state of high alert for hours at stretch, one miss and you might end up dead. Sniping is risky.