It's the old saw again: verisimilitude or game balance?
By all rights, a sniper* SHOULD generally be a "one-hit kill" weapon. That's the entire point - to kill a target from far away, with one shot.
However, from a game balance perspective, having OSKs really sucks, both for NPCs and PCs. Nobody- NOBODY - likes to have a PC taken out from a shooter a half-mile away with no chance to survive or anything.
I think there's a good compromise to be made here that preserves some semblance of both game balance and verisimilitude. Your standard rifle used to OSK a bog-standard human target (who is a soldier) is about a .308, right? And it's about right to say that the average human (who is a soldier) would have about 25hp, right? Well, find a rifle that shoots approximately that round, and tweak the house rules to ensure that that rifle is an OSK against that human (say, make sure it does 27-28 damage, after armor, when sniping, which provides some overkill). Then, take into account that this is a world filled with strange and magical creatures that very often have a LOT more damage resistance than the average human. Sure, it may take a .308 to OSK a standad human, but what about a full-blood Gregori or Anni - whatever? Those might require a .50BMG, and as far as sniper weapons go, they're rare, heavy, expensive, and NOT standard equipment. In essence, game balance is achieved because almost nobody is going to be a sniper in the first place - they almost certainly aren't going to have the "Proper" tool for the job (.50BMG is too heavy and overkill on humans, .308 or smaller will just piss off the nastier races). In essence, snipers actually largely cease to be a major part of the game altogether because the game world makes being a sniper too difficult to make it worthwhile. No mechanical fixes needed. You don't run into snipers outside of places where they'd have major logisitical support (city walls, for example) so they can grab the right gun for the job. Snipers can still rock somebody this way (the sniper rules DO support actual OSKs), but an accurate portrayal of the game universe itself ensures that they don't come out very often, if at all.
*Note that I'm talking about actual snipers (800-meter-plus shooting), NOT "combat marksmen". Those guys are very good shots, but they aren't shooting at unaware targets outside of a military engagement; once they're in a position to shoot, the OPFOR is generally also shooting back and/or taking cover. Thus the whole "OSK on an unsuspecting target" never comes up. Ranges for combat marksmen are standard infantry combat ranges; 25m-200m.
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Or if you MUST have actual "mechanics", at the very least I'd add in it taking a full ROUND action to aim per 300m distance you're shooting. Lining up shots past 300m is going to require you to take into account more ballistics and crosswind issues than shooting at infantry combat ranges. That takes time, even for the pros. Oh, and add an additional Difficulty modifier to any shots past 300 yards (I suggest a +6, +1 per 100 yards or portion thereof past 300) unless you have a spotter who takes a full round action to spot for you (requiring a Rifles/Shotguns check of X - you math people figure something out). It's HARD to shoot crap far away under non-range conditions - this is how it works. Nobody ever has spotters for their snipers in RPGs...