The funny thing is that you can shoot someone with impunity with a soft lead bullet and a revolver, or bolt-action rifle, and leave very little forensic or ballistics evidence. All those other methods still show the aurhorities that the victim was shot by something. If anything, you're simply generating additional mystery that will garner the case more attention and resources put twoards solving it.
Honestly, unless the perpetrator leaves his wallet at the scene, or gets his face on video, 99% of crimes are solved by human information. Who talked to who, who saw who where etc. It would seem to me that unless a fingerprint or DNA is on file, the forensics are more to secure a conviction rather than idenfify the perpetrator.
That's why serial killers rack up such a body count. They don't talk to anybody, at least until they get so squirrely they feel the need to start writing 'zodiac letters' etc. to the papers.
Being aware of your surroundings and who knows what about who seems like a much better way to conduct "wet work" than exotic bullets.