[MadMax]

It's an interesting question I've wanted to solve with empirical tests but never had the time. I've wanted to make fps measurements thru an optoelectronic chrony (ones with the wands) to take fps measurements at different ranges with different weight pellets with the same gun.

I would guess that at very close ranges, the mass of a pellet would not affect impact energy, but heavier pellets would carry higher impact energy at longer ranges.

Perhaps a more useful piece of data would be time to target. Heavier bbs start at a lower muzzle velocity, but carry their speed better. There may be a tradeoff point where one might choose a moderately heavy bb for a given muzzle energy and engagement distance.