For shot the problem is that each piece is very small, so air resistance slows it down meaningfully if the density isn't there (e.g. steel vs lead); so the exac

For shot the problem is that each piece is very small, so air resistance slows it down meaningfully if the density isn't there (e.g. steel vs lead); so the exact same power at the muzzle will have very different power when hitting the bird if a less dense material is used.