IMO the first assassin drones will fire a single .22 short round into the backs of victims’ necks. The drones need only be large and massive enough to carry and fire one .22 short with enough velocity and accuracy to be lethal.
Imagine if the Chicoms sailed a container full of these into the GOM just off the coast. Released a million of them. With just enough ‘intelligence’ to find a humanoid target and fire.
Imagine if they all looked like crows or blackbirds.
Imagine if they knew how to recharge using outdoor electrical outlets and utilize trains and tractor trailer rigs to hitchhike.
You just think the DC sniper caused chaos.
The ‘happy’ part of this scenario is the price range is rapidly decreasing. In 10 years it might not be the Chicoms. It might be the Nicaraguans or Brazilians or Cubans or Andorreans.
Or just plain old Al Qaeda.
Or just some random wealthy wackjob. In China. In Japan. In Bermuda. Wherever.
They'll need to be massive enough to have sufficient inertia that most of the force of the powder going off is imparted to the bullet. If the're too light, the bullet will likely do minimal damage to the target, while the drone ends up a half mile away.....