One shot one kill in airsoft is very difficult, and it's not recommended that anyone join airsoft with being a "sniper" in mind. The maximum effective of sniper rifles are not much greater than that of an Automatic Electric Gun.
Assuming you're around 5'10", go outside and take about 60 full length steps, and that's the typical "long" distance engagement in airsoft with no tail wind or lobbing (pointing upwards and shooting your gun like an artillery piece). Take another 10-20 steps, and you've got the effective range of a top-of-the-line sniper rifle against a stationary target. It doesn't take one very long to cover that difference in effective distance, and that difference in effective distance comes with a significant sticker price for the sniper rifle.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."