The suspected poison, ricin, was cutting edge at the time. It was a top-secret concoction yielded after decades of research in Soviet chemical warfare labs. Scotland Yard was so unfamiliar with this new super-poison that they had to test it on a hapless pig to confirm how it killed. The projectile itself was a modified 1.52 mm jeweler's bearing, normally only used in precision watchmaking. The bullet was relatively harmless to the body, no more damaging than a BB, but the pellet was coated with a special wax that would melt at body temperature, and inside was the new and deadly poison. In fact, the wax worked so covertly that it wasn't actually discovered until investigators found traces on a similar pellet pulled from the body of Bulgarian exile Vladimir Kostov, who suffered, but ultimately lived through the attack.
The Communists killed dissidents with tiny platinum Death Stars. And, somehow, they lost.
That's right: This wasn't an isolated incident. Multiple men have been attacked with a poison watch bearing fired by an umbrella, wielded by a certainly Bulgarian, possibly watch-themed super-assassin.
Batman writers, eat your hearts out. (Seriously, do it quick; it's a way nicer death than ricin poisoning, and the Watchman does not take kindly to copyright infringement.)
Running only amuses him.