Rumpel Felt, you obviously dont understand the very basics of physics. The energy your mechbox puts on the BB is the same no matter what BB weight you use, why do you think they use 0.20's to chrony at games instead of just allowing anything? If you are shooting 400fps with 0.20's than your mechbox is exerting aprox 1.49J of energy, if you fire a 0.25 and it chronies at 370fps you still have that exact same 1.49J of energy. The equation to calculate this is E=mv^2 where m is the mass of the BB in Kg, v is the velocity in m/s, and E is the kinetic energy that that BB has in Joules. If you throw a baseball or a bowling ball with the same amount of force, the baseball goes a hell of a lot faster, doesnt it? Exactly the same thing, all basic elementary physics.
The heavier BB's empart that energy onto objects in their path differently than lighter BB's, I'm not 100% sure how this one works but a 0.25 will shoot a hole through a trees leaf in the same situation where a 0.20 will bounce off of the leaf. No, I'm not saying if you go out right now and shoot a leaf with a 0.20 it will bounce off, I'm saying in a situation where the 0.20 will bounce off, the 0.25 has a much higher chance of punching through.
I remember when I switched to 0.20's I did so because I was using a stock gun so figured I was supposed to. I fired through a VERY sparse bush and all the BB's went everywhere and more than a few came back at me. Just yesterday I was using a stock gun with 0.25's and was able to rip through moderatly dense bush to make kills. I was watching leaves and twigs snap and fly away in the path of my BB's where as when I was using 0.20's the leaves and twigs where just rustling and shaking and it was the BB's that flew everywhere. The differences are VERY noticable in woodland environments. Indoors 0.20's also bounce and ricochet far more than 0.25's, thats why the heavier the BB, the more you can feel the hit even though the energy is the same, its all about kinetic energy transfer.