I've always loved using 0.36g BBs in my KSC G19, even my stock one, easy 120ft+ kills with it. Then again, I look at a BB and it flies off to my target at any distance, even a few provinces over, so I'm not much of one to argue with some of the above posts.
Yes, ok, I was serious in the above statement, have gotten those ranges with my G19, but overall I use strictly 0.28g BBs in all my guns except my sniper rifles (0.30g, and my tests show next to no velocity drop between the 0.28g Bastards and the 0.30gBBs). 0.28g WILL go farther than anything ligher, and even my 360fps MP5 two years ago could clear 200ft with a spread of maybe 2-3ft at that distance. I'm going to be doing new studies in the new year to find out the best velocities in guns regarding performance. I'm not happy with close to 400fps in AEGs, much worse than when set at 360fps, and sniper rifles, 450, 490 and 585fps are all bad performers, but the same rifle (CA M24 fully upgraded) can get 0.30g BBs out to 300ft very easily (mind you, my group size was in the area of about 10ft, but still, considering most BBs I've seen fly veer off sometimes even before 200ft) when set to around 330fps, which is why I started thinking of making some major changes to my gun set ups in the near future to study more. Heavier BBs do require more hop up, equals more rubber in the barrel in the BBs path, and at higher velocities the BBs loses performance and seems to go out of control sooner in the flightpath than a slower moving BB of the same weight.