Et3rnalLegend64 said:
TF2 is the most balanced suggestion, even if it makes no sense whatsoever. Staying scoped charges power and a full charge will body-shot-kill anyone who isn't a Heavy. However, you are vulnerable while scoped so head-shots are safer but more difficult.
You could easily make it make sense, while playing along with the supposed realism mechanics they use in "realistic" war shooters. Just take the whole shaking mechanic and use that as the charge meter-- when scoping, your aim will be unsteady and holding an enemy by the crosshairs will "charge" your shot. Taking a shot before you're fully charged will cause the game to angle the bullet away from your target. Do a quickscope and it'll just flat out say you missed. Fire a bit too quickly, but aim dead on on someone's head, and it'll make it a grazing shot that skins the side of what you can see-- still damaging, but not blowing somebody's brains out. Fully charged, the gun is still, the shot'll go right where you're aiming, and a heart shot/head shot will drop them. That work?
EDIT: Incidentally, most people don't seem to realise this: Sniper rifles are just highly accurate rifles used for aimed shots at range.
They are not automatically more powerful. Most "snipers" are just modified assault rifles with a scope firing single-shot. The whole "snipers are one-hit-kill" thing only makes sense if EVERY gun is one hit kill.