AIRSOFT QUESTIONSSSS!?!?!?!??!?!?

I got a sniper rifle that fires at 470 fps with .12 bb's, 400 fps with .20 bbs, but when i shoot with .20 bb's the bb will go accurate for 100 feet then it will fly straight up into the air. I know if i get heavier bb's that the gun will be amazing, but how heavey should they be