Right. I understand that semi-auto is sometimes unnecessary, but even a single shot seems like it would be better. Anyway, answer below has a nice explanation.

[quote]In Single shot high accuracy situations, a bolt action is still your best bet. Because the action of a semi auto will change the center of gravity midshot, and affect your accuracy.[/quote]

Not all of them are bolt action. Quote from Wikipedia: [spoiler][quote]The choice between bolt-action and semi-automatic (more commonly known as recoil or gas operation) is usually determined by specific requirements of the sniper's role as envisioned in a particular organization, with each design having advantages and disadvantages. For a given cartridge, a bolt-action rifle is cheaper to build and maintain, more reliable, and lighter, due to fewer moving parts in the mechanism. In addition, the absence of uncontrolled automatic cartridge case ejection helped to avoid revealing the firer's position. Semi-automatic weapons can serve both as battle rifle and sniper rifle, and allow for a greater rate (and hence volume) of fire. As such rifles may be modified service rifles, an additional benefit can be commonality of operation with the issued infantry rifle. A bolt action is most commonly used in both military and police roles due to its higher accuracy and ease of maintenance. Anti-materiel applications such as mine clearing and special forces operations tend to use semi-automatics.[/quote][/spoiler]