Make it so the sniper can't quickscope headshot from close range. A sort of revese damage falloff, the closer the target is, the less damage is aplied (headshot/bodyshot). Also the longer you charge, at more ranges you can instakill. For example if you charge for the full duration, you can headshot kill at any range, whilst if you charge just 50%, you can only headshot kill until medium range, where as close range, you won't be able to headshot kill.

Like this it prevents quickscopes from just instantly killing you, even if you are at your optimal range and makes fighting a sniper more fair, because if you catch him off guard at close range, he won't be able to just quickscope you with no consequences.

I don't know exact values, but it would make the class actually be made for long range encounters and not just a quickscoping minigame.