Here's the basic principle of fps....
The more fps you have, the less amount of time it takes for your bb to hit it's target, which helps in fast engagements. But here's the problem, if you're shooting 500fps, you can't use .2s and expect to hit anything past twenty yards. This is due to too much power being exerted on a very tiny round, a .2 bb. Now what do you do? You put in .3s to get some decent accuracy at range. Now that you have so cleverly solved your accuracy problem, you notice that your bb's are now moving much slower than the .2s were and that you have lost your advantage of low delay between the gun firing and the bb hitting it's target, now you're stuck where you were when you had 400fps and your gearbox parts won't last as long as they normally would have if you hadn't stuck your M150 in it to improve delay time and range. That and the fact that you are now spending more money on 500bb's that you were on 2,000bb's of the lower weight, should be enough to discourage the guy who wants crazy fps. You will also notice that your range hasn't changed much, since you're using a heavier bb, more energy is needed to propel it to a velocity lower than the original gun was before you had upgraded it. So, in the end, the only thing you gain a lot of when you upgrade the velocity of your gun is, slightly more range from the more energy being applied to the bb that has more weight and momentum, now that you have spent hundreds upgrading your gun, you see a very small difference in performance. Will there ever be a way to increase a guns range far beyond the barrier of what is now considered the maximum range for an airsoft gun using 6mm ammo? That's what I hoped to find out with this post, and hopefully spark a few good ideas.

I agree with Soccer on this one. I've been working on several Hop up experiments that have lead me to believe that the range of airsoft guns is not limited only by the bb, but also by the hop up. You can find people wanting better range, so they get new hop ups and it works. But that's info for another time, I still have some experiments to do. :D

But also, as several people have pointed out. A heaver bb is traveling faster down range than a lighter bb, because it is heaver and keeps it's momentum for longer. My 500fps DMR is able to make 300ft shots, and the difference between .3s and .4s in the gun was accuracy. They both took the same time to hit the target, but the .4g where much more accurate.