It all has to do with joules. A heavier round will always have more force or energy than a lighter one. An AEG is spring powered, so the thought is that as you increase BB weight you bring down the fps, but the energy won't change drastically due to this fact. In a GBBR you are using compressed gas to project the BB down the bore. So the gas will expand at a certain rate no matter how much heavier the BB is. So instead of seeing a huge fps change from using a heavier round your more likely to see a jump in joules if force. And joules is what can really hurt someone. Each fields have or may have it's own fps/joules limits. And they can calculate the energy output of a projectile by knowing it's velocity and projectile weight. I hope this helps. There are many posts within ASC that can give more detailed information on what to look for.
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