What the guru said.
It just comes with experience. There are some cheats to get around it; some scopes have range brackets based on a certain height of person. Some scopes have laser range finders.
300ft is REALLY hard to do. With my VSR shooting at it's optimum (430fps on .30g BB), in a 0 wind environment, I was able to get the BBs out 280 feet, but I'd only be comfortable shooting out to 220 feet accurately.
Being a sniper in airsoft does have it's advantages. It's a delicate balance between marksmanship, stealth, and SILENCE.
You can sneak behind a squad of guys, take your shot and take out 1 or 2 guys before they figure out where you are and lay down some heavy full auto fire. Seeing as many guns I've upgraded are able to range sniper rifles, but just not nearly as accurate.
HOWEVER, if your rifle is as quiet as a gopher fart, you stand a chance of taking them all out from 50 feet away and they'll never hear you
Back when everything was still new, I could shoot from 20 feet, and you COULD NOT hear my rifle. Racking my bolt was way louder than the shot being fired lol