The idea of a very frangible (untraceable after firing) bullet really depends on the shooter wanting to retain the weapon. I always liked the totally impractical bullet made of ice.
Seems to me a professional has enough access to weapons that disposing of each weapon after it is used is preferable. On the other hand, if someone really wants to retain their weapon, for whatever reason, and is concerned about ballistics matching there is a relatively new breed of revolvers using .410 shotgun shells (e.g. Taurus Judge), whose shot will have no rifled marking to match. I view those as an odd novelty item but they have been reportedly selling well. Of course you're also back to a revolver which is harder to silence.
Those old Nagant revolvers are very cool but definitely a 'signature' weapon. Put me in the group suggesting they are rarely used because of their odd ammunition. Okay, they will certainly be more popular than Gyrojet guns.
