The game currently has two snipers and they work differently.
Widowmaker
Widowmaker has a grappling hook that she can use to pull herself up to ledges and rooftops. You should use this to position yourself somewhere where it's hard to see you.
Her alt-fire will use her zoom scope. When zoomed in, a power meter appears on your screen. When the power meter is full (takes about 2 seconds), headshots will do 300 damage and body shots will do 150 damage.
Keep in mind that a few characters have a different head position:
- D.va's head is...
- Mech: The entire cockpit she sits in.
- Girl: Her head
- Bastion's head is...
- Recon configuration: His head
- Sentry configuration: The glowing spot on the back of the turret near the top.
- Tank configuration: The glowing spot on the back of the tank near the top. (This used to be at the top of the tank, but I'm pretty sure that changed during beta.)
Bear in mind that death replays as well as your sniper rifle's tracers (the red lines that show where you were shooting) give away your position, so you periodically need to move.
Don't forget to set your venom mine down somewhere. It's best use is as an early warning that an enemy is nearby as it shows on your screen when it's triggered.
Your ultimate should most likely be used shortly after you get it as it shows all enemy positions to your team.
Hanzo
Unlike Widowmaker, Hanzo doesn't have a zoom scope. As such, you tend to do better when you're closer to an enemy. Constant movement between shots helps quite a bit.
Remember that you can climb walls and use this to your advantage.
Hanzo's arrow do more damage if you charge the bow for a second before letting go of the attack button.
You should use your Sonic Arrow whenever it's ready to help show your allies where enemy players are. Keep in mind that, unlike Widowmaker Ult, it only shows enemies near the arrow.
On Defense, consider shooting it at one of the enemy doors during setup time so that you can see what heroes the other team has.
Your Scatter Arrow is a mixed bag. It's most powerful when shot in a small room as the shots ricochet off walls. However, you can still do a lot of damage with it by shooting the floor directly in front of an enemy.
Your ultimate is incredibly powerful as it sends the twin dragons/wolves in a direct line in front of you and travels a good distance, but bear in mind that it won't hit anyone within about 5 meters of you.
Turrets: these seem pretty strait forward but can actually be used in a variety of ways. Obviously the most strait forward is to cover choke points. This is extremely effective IF you can place them in such a way, that is inconvenient for the enemy to have to destroy. For example, if you place turrets above the enemy, on to your side of the wall, when they walk through they are forced to look up to shoot it. They are using that time to look up to shoot the turret and not your team. That's a minor victory in my book. How I like to use the turrets however is a bit different, and I'll tell you why. Symmetra gets notifications when her turrets are attacking, or when they get destroyed. I generally place 3 at the choke point if I'm defending, and I'll place the remaining three around the area where sneak attacks are common. This allows me to spot the attack well ahead of time and warn my team mates. Do not under estimate this, it's extremely useful. Also they slow, so there is that. I should also note that the turrets can be placed ON payloads, so when attacking you get mobile turrets, which can be nice. (I don't cover turret placement itself very well here, that's a topic in and of itself. however there are good tips from other commenters below)
Source: https://www.reddit.com/r/OverwatchUniversity/comments/4lmr4u/why_symmetra_is_amazing_and_how_to_use_her_kit/
Don't hesitate to read the Reddit post, you have many more information about the rest of Symmetra's kit and the synergy between her abilities !
Best Answer
The Turret needs to be upgraded to level 2 before it will turn into a level 3 turret. As soon as it hits level 2 it instantly upgrades to level 3 if Molten Core is active.