Have you tested your sniper rifle to ensure that it is capable of hitting a target at 300+ft? Ensuring that your rifle can hit the target at that kind of a distance is the first step. I assume you have, i just wanted to make sure. You didn't mention what type of gun you have and the FPS its currently shooting at so i like to take that into consideration.
A BB is greatly effected by wind and other environmental factors. When shooting to distances of 300 yards, getting an accurate shot at that range is difficult.