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.
[X] Plan: Nighttime Heart Attack