Slingshots arent bad, it all depends on campaign. I can see a lot of homeless bums knocking out devilrats with them (See the new movie, coming to theaters near you in 2074 - Troll Hobo With A Slingshot, starring Roofter Howitzer).
The question is how flexible the weapon is in regards to ammunition. Capsule rounds are the first ones that comes to mind (which would essentially be paintball rounds).
A modern slingshot can be VERY dangerous, now imagine it in the hands of a moderate strength troll - that will be the same damage output as a heavy pistol. It also completely silent.
We also have the luxury of smartlinks for only 30Y, which makes it even more funny as the external version is 400Y. An underbarrel weight would be possible as well since it would be a stabilizer mounted at the bottom of the handle - but it becomes a bit expensive if we start to look towards the gyrostabilizer.
Possible Ammunition
Capsule Rounds: As above, we use paintball rounds filled with whatever we like.
Ex-Explosive Rounds: Capsule round filled with impact detonated explosive
Frangible Rounds: Glass beads that shatter on impact
Silver bullet: Silver ball (duh)
Hi-C Plastic Round: yes, well, plastic balls, less impact etc...
And perhaps we should allow it to be armour piercing...
Slingshot VS Coconut