>the possibility of extremely cheap drones that approach the victim's face very quickly and pierce a bolt into their brain through one of their eyes.
That's super impractical for many reasons. Drones don't move fast enough, nor can they change their velocity quickly enough to do that. People will almost certainly react if they see the drone coming for them, and cover their face, or swat it down, etc.
But even if it did work, it's not a serious threat. For some reason people spend a ton of time thinking about all the ways new technologies could be abused by terrorists. For some reason they never consider that tons of existing technologies can be abused too.
Many people have come up with really effective terrorist ideas that would kill lots of people, or do lots of damage. The reason the world is still here is because terrorists are really rare, and really incompetent.
> Short range slingshot mechanisms are several orders of magnitude cheaper to build than firearms.
Not necessarily. It's actually not that difficult to make a firearm from simple tools and parts from a hardware store. And it will be way more deadly and accurate than a sling. Not to mention simple pipe bombs and stuff.