I don't actually know right now, if it won it would take a series of very concerning Google searches to come up with an answer to what exactly would happen if you dump a sling stone's worth of joules directly into a human thoracic cavity, but I'm pretty sure the eventual answer is going to be grisly. That research could be done by someone in the thread ahead of time if you want to be sure.
So I've done a little research.
If it's actually modeled as a sling stone, not necessarily a kill:
View: https://www.youtube.com/watch?v=aGFCR5oDjKI
Look at how small the wound channel is, and knowing the sling bullet would disappear, it would be like inserting a skewer somewhere in the chest. Obviously bad, but also survivable if you get lucky. It depends all on where it hits.
For just dumping the joules generally, it'd be a
blunt cardiac injury. And people seem to survive those, but not always.
Because there's not a ton of joules in a bullet. Using
this for speed and weight, 100mph and 1 oz, we plug this into
Wolfram Alpha to get 28.33 Joules, which again,
Wolfram Alpha tells us is a little more than the amount of work done to compress a bike spring 5cm. We then spread this work around the thoracic cavity, and I think you come off alive, but in pain. Sling stones don't seem to have a ton of energy, but they have enough to put someone down for the battle, and that's enough for military work.
And from this, I don't think I can endorse a Dart Attack. Too little damage, with a fail state of the dude surviving.
EDIT: a trained boxer does about 700 - 1000 Joules/punch (presumably with gloves and no fear of a hurt hand). A karate person does 100-450 with bare fists. So a Karate artist doing a weak punch at half speed (velocity is squared) is roughly equivalent to a sling dart.
[X] Plan: Nighttime Heart Attack