A tachyon is almost indistinguishable from a slower-than-light particle. It would take just as much energy to "slow" it down towards light speed as it would take to speed up a normal particle towards light speed.
The only notable difference is that the tachyon's relative movement through time would appear to us to be going backwards in time. (And a photon traveling at the speed of light experiences no time; its entire existence happens at the same time, so far as the photon is concerned.)
So who's to say your tachyonic bullet did not in fact jump from the victim's body into your gun?
Well, causation is the big problem there. The chemical reaction in the gun caused the bullet to shoot out.
So look for something that appears to have been caused by what happened at the end, and ta da, there's your tachyon.
(I guess that explains all those "the end justifies the means" people out there.)
