Hey I just noticed something. On 007 it says the KJW M700 is a Firearm and requires a "FAL" to buy. I was under the impression that because it does not meet the energy requirements to be a firearm that it's an "unclassified firearm" (basically like a 4.5mm pellet gun).
Also yes I didn't catch that, stryak is right you can't use a heavier BB to offset the power, power and momentum is constant in this case, what changes is the speed of the projectile against weight.
I'm guessing you're probably not a Physics or math major then right? I'm going to go with the guess that you're a social science or humanities major
. (Just poking a bit of fun at the non science majors. Hell, technically I'm a social science major because of my path in my major (Human/Cultural Geography/Demographics)).