Delve a yard deeper...
Bio-degradable ammo to the extent of passing through a bird's gut or quickly dissolving from exposure to the elements means that murder will be harder to solve.
How can our CSI heroes bring closure to victims' families when the bullets used to murder are no longer evidence? I think this whole episode is a conception process of a future American "cheka, NKVD, KGB" monster in the making. Even the Nazis could figure out who the monsters were of Katyn forrest. Future regimes won't even be able to hide their wicked deeds in the depths of the sea.
Frangible bullets could be an option, though they would ruin a lot of meat. But as you point out, they're impossible to trace. They go *POOF!* when they hit and leave nothing but a little sandpile. Let's see ol' Gil make heads or tails of that!