People are better than we give ourselves credit for. I remember working at Disneyland during 9/11, the FBI told us we were considered the 4th most likely target in the United States, and mandatory bag checks at the gates made the lines longer and slower than they'd ever been, so enormous crowds, thousands of people, were gathered and effectively trapped between Harbor Blvd and the actual gates. I couldn't help but think how easy it would be for a few guys or even just one to gun down hundreds with a couple of uzis or throw a few pipe bombs into the crowd. Nobody ever did. That Las Vegas concert shooting was the closest anyone has ever come. There are probably many vulnerabilities like that. Poison the water supply to a hospital. Close off a mine on purpose. My girlfriend's brother back in 2004 managed to drive his speedboat with me in it through the LA harbor, right alongside container ships, and succeeded in evading the Coast Guard. It was just a joyride to him, but someone more malicious could probably sink a container ship or even an oil tanker.

These things almost never happen because people just aren't that evil, at least most of the time. When it happens, it's an extraordinary event.

But the tooling required for guns and rifling in particular is pretty hard to come by, whereas the tooling required to assemble a DIY drone is basically indistinguishable from R/C hobby equipment. You can (try) to regulate where/who/how/when can fly a drone, but regulating drone building seems almost impossible to me.

You can setup a machine gun on a Remote Weapon Station and control it via the internet. You could have agents as machine gunners. Same with rocket launchers.

We are literally seeing the large scale use of robotic weapons platforms AT SCALE.

Machine guns create a storm of ballistic objects, and have been doing so for over 100 years. Large scale murder isn't new. We just have systems that have cameras that make it easy to put the footage online now.

In your basement with simple tools. 3D printers made it a lot easier and better. You can make a pipegun with very little skill.

Ammunition is a bit harder, but again, there is criminal markets that fill the needs.