So I have a head just FULL TO BURSTING with angry hornets today.
Improvement on the bullet bot
Instead of designing a motor somehow intended to pierce through just design a shell to be penetrative enough to punch in, maybe burrow a bit, and upon hitting a certain depth or something resistant like bone mass trigger a few hundred milligrams of absolutely top shelf high explosive wrapped in a bit of very high tensile strength, lightweight plastic or ceramic.
You might even design it to deploy barbs and arm, but wait to be jostled or perhaps exposed to light, making it quite inoperable unless disarmed. This is the popbot or more vulgarly a splatter bug.
The same principal could be deployed en mass for area denial like a flying mine that eats light to stay powered and occasionally move about a bit in an area. Make them big enough for an extremely meaningful concussive blast and have them detect for heat and or a human outline, fly up at great speed, land, lock into the surface of who they land on, and become hypervolitile. You can't pull it(more likely them) off or it will blow. It's too small to disarm even if you got past the volitiliy somehow. It's on you now amd worst of all emitting a distinctive dissonant noise to instill panic and perhaps raise an alarm. Worst of all it will compromise an NBC suit, and even an EOD suit can't protect you from hundreds of concussive blasts. Use rfid to make someone a non target
I call them the locust.