Dps Carry
Due to his low hp, sniper always stand behind his teammate to attack the enemy. You have to put yourself in a good position wherever you are in a team fight.

Sniper has one of the best attack animation in dota. But due to his low base damage you might find it a bit hard to attack

Pros
[+]Long attack range
[+]Hard to die in team fight if you position yourself well
[+]Good attack animation
[+]Has a mordern weapon (SNIPER)
Cons
[-]No escape mechanism
[-]Low health early game
[-]Once you are out of position you are basically dead

Level 1- Take Aim
Level 2- Shrapnel
Level 3- Take Aim
Level 4- Head Shot
Level 5- Take Aim
Level 6- Assassinate
Level 7- Take Aim
Level 8- Shrapnel
Level 9- Shrapnel
Level 10- Shrapnel
Level 11- Assassinate
Level 12- Head Shot
Level 13- Head Shot
Level 14- Head Shot
Level 15- Stats
Level 16- Assassinate
Take aim is to be max for obvious reason(To increase the attack range of sniper.So you can stand back and farm and not get initiated).Taking single level of head shot is sufficient. chance to stun only increases by 5% per level.
Another skill build i usually see is max shrapnel by 7. Go for this build if you and your teammate want to kill the enemy
Level 1- Shrapnel
Level 2- Head Shot
Level 3- Shrapnel
Level 4- Take Aim
Level 5- Shrapnel
Level 6- Assassinate
Level 7- Shrapnel
Level 8- Take Aim
Level 9- Take Aim
Level 10- Take Aim
Level 11- Assassinate
Level 12- Head Shot
Level 13- Head Shot
Level 14- Head Shot
Level 15- Stats
Level 16- Assassinate

Shrapnel

Fires a ball full of shrapnel into the air. which promptly explodes, showering the target area in explosive pellets. Enemy units that walk in this area get damaged and slowed. Deals 1/3 damage to buildings. Lasts for 8 seconds
Cooldown: 15 sec
Mana Cost: 120
AoE: 350
Duration: 8sec
Effects: 15% Slow,12 DPS/20% Slow,24 DPS/25% Slow,36 DPS,30% Slow,48 DPS
Sniper primary ganging tools.It has a 350 aoe that can slow enemy in a group.This spell can make sniper useful in early game slowing in a group is no joke.This can affect tower too.Before level 6 try to use it on tower if your mana pool is full so you would not waste the mana regen.

HeadShot

A shot to the head deals bonus damage and stuns for a short duration.
Effects:
25% chance to deal 30 damage and ministun
30% chance to deal 40 damage and ministun
35% chance to deal 50 damage and ministun
40% chance to deal 60 damage and ministun
Duration 0.01/0.1/0.2/0.2
This spell let you cancel channeling skill.Since sniper has far range he can almost cancel quite a number of channeling spell such as engima(black hole) bane(fiend grip)Rhasta(Shackles).It can also cancel tp

Take aim

Kardel uses his dwarf engineering skills to make modifications to his rifle, enhancing the range of his rifle.
Effects:Attack Range +65/130/195/260
This skill is the main reason why you pick sniper. With its long range and good positioning you are basically untouchable.

Assassinate

Take time to draw a bead, and then deal a large amount of damage to the target at a huge range.
Mana cost: 175/275/375
Cool down: 20/15/10
Casting range 1500/2000/2500
Effects 355/505/655 damage
One of the highest nuke damage for carry hero.This skill can help you chase down fleeing hero.In team fight if nobody is escaping just try to right click and save your ulti to kill fleeing hero