I just recenly bought a famas f1 CYBERGUN foreigh legion aeg airsoft gun and it says 466 fps with 12 gram bbs but then 20 gram bbs is like 360.like whats the difference.Why does it make my gun less more powerfull and which bb should i use and is it okay if i shot already 2000 .12 gram bbs is that like way to much or thats fine but dont do it anymore and will that hurt my gun.