Neat FYI: http://www.airsoftcanada.com/showthread.php?t=51752
I took every chrono reading in that thread, threw them into Excel and worked out the energy for each, then averaged them for each BB weight and data set. I posted my conlusion somewhere here but the jist of it was that as you increase your BB weight, the total kinetic energy (minus rotational) increased by around 5% for the same gun - EXCEPT the .28's always wanted to decrease slightly for some reason.
The pain vs weight question is also something that has boggled my mind. With a quick test we were doing one day we took a 400fps gun and a 370fps gun and tested them at around an average to long typical airsoft engagement distance (probably around 100+' but I'm terrible with distances). The .25's on the 400fps gun hit noticeably harder and the .23's on the 370fps gun hit noticeably harder. Probably around a dozen shots or so with each weight at each fps and on the legs
Ever since I did that test, I've been skeptical and want to do some more maybe this winter or next season (note, test was also confirmed by a buddy as we took turns shooting each other).
I'd like to hear someone who knows what they're talking about talk about pain and what factors are involved. I don't think simply saying KE without any proof or reason is good enough; the body is a pretty complex system.
There's also this to look over: http://cybersloth.org/airsoft/trajectory/index.htm
I looked over his theory and it seemed pretty sound (I had a few qualms) however his lack of empirical evidence (which he claims he has) to back up his neat little theory was sketchy. Seemed to me like a typical engineer...if it doesn't fit your theory, you fudge things to make it look like you know what you're talking about so you don't appear to be wrong.