Real life sniping
A real life sniper would only engage his targets when he has a ballistic advantage (over 400 meters of open terrain in between him and the target). The sniper would take his shot from a well concealed and fortified FFP - final firing position.
This accomplishes 2 things - the enemy cannot effectively return fire and cannot charge the sniper's position due to the risk of heavy casualties.
Real life snipers operate with a spotter and get in position long before the enemy passes along the potential line of drift.
Airsoft sniping - its Tough!
When you bring an airsoft sniper rifle to an airsoft field, you are facing many challenges:
You are not exempt from the game's rules and usually cannot insert before the game to find a good position. This causes you to rush in with the rest of your team. Ever tried running in Ghille? Not fun!
Airsoft fields have ridiculous amounts of full cover that you cannot shoot through. This makes airsoft a lot harder than the real life sniping.
Airsoft BBs, even at 0.4 gram are still easily deflected by leaves, small twigs and dense forest undergrowth. This is especially insidious in the fall when a "clear" shot will hit an invisible wall of twigs in mid flight.
Stock airsoft sniper rifles do not have the accuracy required for sniping. Unless you spend 300-700$ on professional upgrades, you will not achieve good accuracy
You rarely have the ballistic advantage that a real sniper seeks. Most airsoft fields have too much cover to allow for clear 250 feet shots. Believe me, I've been there. A lot of fields limit the FPS on sniper rifles to 450-500 with 0.2s. I've never seen a field that allowed hotter rifles.
Finally, most would be "snipers" don't have the patience to play as a sniper. Missing hours of gameplay as you wait for that one shot that may miss turns a lot of people off the airsoft sniper role.
Airsoft sniper rifle varieties
There are 3 types of sniper rifles on the market today, not just the utg l96 AWP - I rule in counter strike (this is why I got mine ) kind.
Airsoft spring sniper rifles. These are souped up versions of spring rifles and usually feature a top rail mount for a scope. The above mentioned UTG L96 comes with about 450 FPS with 0.2s out of the box with medium accuracy. Putting in a tighter spring may make it really hard to pull on the bolt and will reduce your rate of fire.
Airsoft gas sniper rifles. These use compressed gas, stored in a magazine or a detachable tank to propel BBs. sniper rifles with an extrenal gas tank are more accurate, as it is easier to adjust hopup and get a consistent muzzle velocity. Sniper rifles with gas magazines feature varying velocity. In most cases, you still have to pull on the bolt in between each shot.
Airsoft electric sniper rifles. There are a lot of "electric sniper rifles" sold out there, but they are pretty much just single shot AEGs with smaller magazines. Getting an AEG to fire at 450-500 FPS requires a lot of mechanical wizardry and constant maintenance. It is tough and very expensive (500-600$) total.