Reminds me of the pen invented in WWII that fired a phonograph needle. Used by Allied underground in France.
Then there was a claim a weapon could be made to shoot ice which would melt after killing the victim leaving no trace. Never heard anymore about it.
I imagine there are things out there we have no idea about.
And how would that work? It has to cause trauma to kill. If an ice bullet, the guy would die of a gunshot wound with no bullet for examination, but the cause of death wouldn't be in doubt, there would be a hole.