Oh....kay....
Personally, I think an excellent implementation of marksmanship is the crossbow in Half-Life 2.
[img_inline caption="Easy to use, ready to Fire and Oh-So-Dandy!"]http://images.wikia.com/half-life/en/images/7/71/HL2Crossbow.jpg[/img_inline]
It's very simple to snipe, but for long range, you have to account for target movement and the ballistics of the bolt because of its comparatively lower muzzle velocity. Also, the .357 Pistol does well too, as a well placed shot does excellently, rewarding those with a steady mouse, because you can't fire while zoomed.