Originally Posted by yuhaoyang
.25g is probably still the best compromise. However, if your gun ends up with a tightbore, it may favor certain brands over others. For instance, KSC perfects have a slightly smaller diameter than a higher grade Japanese BB (Azathoth gave them to me... I forgot the brand), thus, those will go through my madbull 6.03 while the more expensive ones won't. Try them all out.
For me, KSC / Velocity Arms jam in every one of my guns that has a 6.03 tightbore. Don't have an issue with regular barrels or 6.04 for some reason though. The only BBs I can confirm work flawlessly in all my guns are BB Bastard. Your mileage may vary though.