Tasks of the sniper rifles are obvious : destroying of valuable targets from an enormous distance using as few ammunition as possible. In most cases target means human target, for example an enemy soldier, a criminal, a terrorist, or a politician.But lightly armored radar or radio stations, planes on ground, helicopters, vehicles also can be destroyed or disabled with larger caliber guns. Range of operation varies between 100-2000m.

Sniper rifles have 3 main category : Military, Police/Law Enforcement, Special purpose rifles

MILITARY SNIPER RIFLES

Naturally used by soldiers all over the world. First demand on these rifles - besides a good accuracy and large effective range - is the light weight, because a mission can be very long, and the sniper must carry all the equipments along with the rifle for hours.It has to be very reliable, has to be same accurate under any weather and climatic circumstances. Easy disassembling and repairing on the field is another important demand. They need a backup iron sigh too. And in the end, it must use military ammunition, as the common 7.62 NATO or the special sniping 7.62x54mm R. The usual range in military/war situations is 700-800m for one shot kill of a standidng human sized target. One can make the effective range beyond 1000m with the usage of the proper, specially designed ammunition. These weapons are divided into two groups :
- Sniper rifles : all the usual sniper rifles. Their task is to carry on single, aimed shots from great distances.
- DMR (Designated Marksman Rifles): These weapons gives accurate support fire to the troops. While the real sniper rifles are bolt action ones for greater accuracy, these are usually semi-automatics, because of higher rate of fire.

POLICE/LAW ENFORCEMENT SNIPER RIFLES

While in a war a wounded target is as good (sometimes better ) as a killed one, it isn't true in counterterrorist or police scenarios. A wounded terrorist means a much more danger to hostages and innocent bystanders. So, a police or CT sniper must not only kill the criminal with one shot, but they have to shot at a certain body part, head, a weapon holding hand,etc...
To make a long story short, thats why those rifles must be extreme accurate, but within a much shorter range. The typical range is usually smaller than 300 m sometimes it drops even below 100m.One more demand, police rifles must be also very reliable under all weather circumstances.

SPECIAL PURPOSE SNIPER RIFLES

- Large caliber sniper rifles : .For ultra-long range sniping, or anti-vehicle usage.They usually shot heavy machinegun cartridges, like the .50 BMG( Browning Machine Gun) or the 12.7x108mm.Effective range of these rifles is up to 2000m with the proper ammunition. HMG ammunition occurs worse accuracy, but recently there are some .50 ammo designed for sniper roles.

- Silenced sniper rifles : needed in covert/stealth missions. They use sub-sonic rounds, and equipped with integral or removable sound/flash supressor. This makes harder for the enemy to discover the operatives. Though sub-sonic rounds decrease effective range to 300-400m, it worth it, because at night gunshot can't be heard within 200 m, and in urban operations this range is 30-60 m because of traffic noises (by day)

ACCURACY, OR WHAT IS THE MOA ?

The most universal way to determine accuracy is to measure the average size of the circle that can be drawn around the bullet holes in the target. Usually the shooter fires 3 or 5 rounds, measure the circle around each groups. In the end, average group diameter determines accuracy. And again, what is MOA ? It is Minute Of Angle. Below on the picture you can see what does 1 MOA means. To make it much more clear, 1 MOA means 1 inch scattering at 100 yards, 2 inch at 200 yards and so on.

So, if we know that a gun can shot 0.5 MOA, than we know it means 5 inch scatter at 1000 yard.A good sniper rifle can shot 1 MOA.