[sargon]

Then there was a claim a weapon could be made to shoot ice which would melt after killing the victim leaving no trace.

That idea is mentioned at the beginning of the 1975 movie "Three Days of the Condor" starring Robert Redford...

[xone]

I’m familiar with the ice bullet concept, but the only ‘not a trace’ would be a lack of a slug, not a cause of death. If it kills you, there is a trace. A natural occurring substance, adrenaline, acetylcholine, can all be used ‘without a trace’ if the autopsy is less than thorough.

Death by impact always leaves a trace whether the impact producer is found or not.