Hmm I see the science but at the CQB distances at my site I don't think it makes much difference. I've literally have my muzzle in a guys chest when we both tried to go around the same corner. Fun story he didn't take the hit which was me actually firing as we both had the drop on each other.A lighter BB will lose energy faster. A heavier BB will maintain energy for longer. So both leave the barrel at 1 joule, but the lighter BB might hit you at say 0.5j but the heavier one will have 0.8, let's say. (These aren't exact scientific figures, but just examples)