The Sniper setup is one of the core archetypal tank setups in World of Tanks alongside Brawlers, Flankers, and Scouts. Most elements of a sniping tank seem second nature to experienced players but newer tankers unfamiliar with the finer aspects of game mechanics will find themselves in dire straits when they thought themselves safe from view, or fall victim to invisible assailants with alarming frequency. There are also certain tank and gun combinations that lend themselves to sniping and others that will never make adequate sniping platforms no matter how much specialized crew training and gear you use.The two foremost elements of effective sniping are accuracy and view range.
Weapon accuracy is the first concern in sniping setups as an inaccurate weapon is unlikely to hit your target at all at the ranges you will be engaging. While Howitzers will occasionally get a hilarious hail mary hit, their shell velocity and high dispersion mean that even fully aimed they will never compare to a standard long-gun firing AP shells. This is purely a square peg, round hole problem. Not all tanks have a choice of functional gun or howitzer and not all rifled guns are even accurate enough to snipe with. The D5T-85BM, at .37m/100m dispersion is about the upper end of acceptable sniping guns whereas the ZiS-4, at .34 is perfect and can reliably hit targets out to maximum range with relative ease, provided you lead them properly. The problem with dispersion is your reticle will only shrink so far. While crew skill and other factors do improve maximum accuracy and aim time there is a noticable difference between a .34m fully aimed gun and a .40m fully aimed gun so if your tanks best gun option has a .38 or higher dispersion you may want to reconsider the tanks role. Even the KwK 40 L/48 at .39m begins to have issues reliably tagging targets at range because of how the server throws your shells.
The primary reason you want to insist on highly accurate weapons is the RNG. While most shells will by definition fall within one standard deviation of your aim point you do not want to have to rely on the vagaries of the deviation engine to place shots on target for you. To achieve success it's necessary to marry skilled play with tanks maximized to your purpose. You want to stack the deck as much in your favor as possible so taking a gun that maximizes precision is key, even if it doesn't hit very hard.
Unfortunately there aren’t many tanks with guns as accurate as the ZiS-4 available and when you hit the big .40 you’re going to be wasting more shells than not at long ranges. The thing to remember is you’re dispersion grows by that many meters in diameter every 100 meters of range between you and the target. As noted in the View Range Battle Mechanics section of the Wiki, the view range of a tank is technically unlimited and the maximum any tank can reach as of the last revision is 598.8 meters with all modules and crew skills in your favor. Rounding that up to 600 meters for easy math your .40 meter dispersion becomes 2.4 meters in diameter at 600 meters range. This is somewhat misleading as the dispersion describes two standard deviations from dead center, since 95.45% of your shells should land in that circle. Your reticle actually shows three standard deviations so it is 1.5 times the diameter of that dispersion value. Since only ~5% of your shells will land outside the dispersion diameter we can safely use that dispersion to describe the problems you’ll face rather than the reticle.
Lets put this in perspective then. Just how big is a 2.4 meter diameter circle? Comparing to tanks, which is really the only comparison that matters here, the T-34 is 3 meters wide and 2.45 meters high so it sounds like you could hit that regularly from the front at 600 meters with a .40 meter dispersion gun. The caveat is there’s a lot of open space to either side of the turret and underneath the hull that’s nothing but air and is still inside that 2.4 meter diameter. Your actual chance of hitting it may be closer to 50% than 95.45%, and you will not be able to aim for weak points with any degree of precision. Considering turret and target movement and the aim prediction you must make prior to firing to account for shell velocity (these aren’t lasers) even with a .40 gun you’re going to have difficulty.
While the 600 meter example makes for easy math, even the perfect Patton detailed in the Battle Mechanics example doesn’t quite reach that with all the best equipment. While closer targets means they are easier to place shots on, it also decreases the number of shots you can fire before they’re right on top of you. Tanks with poor view range also make poor snipers because an unseen target is very difficult to hit. Teammates closer to the fray cannot be relied upon to survive indefinitely and server updates on obscured targets aren’t very fast, meaning the enemy will have ample time to dart between openings before it even renders if you’re relying on a teammate to spot for you. You’re going to end up doing a good deal of your own spotting when you’re a sniper.
If you are in stealth yourself you will lose most of that bonus when firing thus a long view range is essential in remaining hidden while firing as a closer enemy also has a better chance of detecting you back when fired upon. It is worth mentioning that view range determines detection range but they are not the same thing. View range is actually a pretty terrible label for that statistic since the server rendering rage has nothing to do with your tanks optics. The maximum range an enemy detected by any teammate will be rendered varies depending on the angle you’re viewing them from due to mathematical shortcuts in the server code. Using the edge of this maximum draw distance to provide your own stealth by keeping other enemies out of your server horizon will cause targets to blink into and out of rendering range as the melee progresses so "range tanking" while you snipe has its own set of issues. The server horison is due to be raised as noted by Serb since there’s no reason you should be able to see enemies at 600 meters on one map and nearly 850 meters on another just because the second map has you approaching each other on a diagonal for example.
Your detection range is your view range reduced by the target tanks stealth rating, their crew training, camouflage paint, camouflage net if active, and intervening cover. The range at which you can detect the enemy yourself and remain hidden while firing upon them is rather narrow. If your view range isn’t artificially boosted by crew training and equipment that margin likely won’t exist at all even with cover.
Basic view range is based on the tanks turret and is increased by the Recon skill for Commanders, Situational Awareness for Radio Operators, Ventilation boosting their skills, and either Coated Optics or Binocular Telescopes. While you can equip both a Binocular Telescope and Coated optics the effects do not stack. Premium consumables can boost this a little further but if you’re paying for those each battle for sharper vision in randoms you’ve got problems. Best to save them for your tier 10 brawlers.