Ok, watch this video but don't listen to the narration. The guy that did it knows nothing of ballistics or how gravity works... In short terms, he aint no physicist so stop listening when he speaks.
The point of the exercise is to show the accuracy of .25, .28, and .30 BBs from the same gun. The gun used was KWA G36C which puts .20s out at about 400fps. This drops the .25 to roughly 355fps, the .28 to roughly 322fps, and the .30 to around 310fps. Slow yes, but watch the video.
http://www.youtube.com/watch?v=ReLpwcl_ ... ure=g-vrec