If you make a piece of dust of about 1microgram fly to someone at about 99% of c, or about 290’000km/s, with Ek=(1/2)mv², we get an energy equivalent to 42’050’000J, or about 10kg of TNT.

The dust would probably vaporise instantaneously, so it would be the resulting explosion that would be deadly if you fired at point blank range.

But first find a dust accelerator that can get enough power for that.