Doesn't the heavier pellet retain more energy over distance? Must be something to do with inertia. That's my guess. Inertia.....or momentum....... no, inertia.
Yup it does retain more energy (not a lot more though) when you get to .25 the max distance drops btw, but I cant remember what the terminal energy was now (lappy packed away again and I never got round to running chairgun under linux )