The FAA reiterates that it is "illegal to operate a drone with a dangerous weapon attached." The FAA is well aware of the videos that have been circulating around the internet promoting such dangerous activities and warns against attaching "guns, bombs, fireworks, flamethrowers, and other dangerous items".
Besides the fact that such items represent a serious danger to the public at large, the FAA also flippantly adds that it will hit you right in your wallet if you're caught with such weaponry attached to your aerial devices. In case you didn't already know, such activities are already outlawed via Section 363 of the 2018 FAA Reauthorization Act.
Under this provision, drone operators can be fined a civil penalty of up to $25,000 per violation. The only way that such activities are permitted is if they are "authorized by the Administrator", but we can't think of any possible situation where this would apply to regular civilians.
After all, the Department of Defense likes to reserve weapons-laden drones for U.S. Air Force and U.S. Army, which operates a vast fleet of aircraft that loiter over the foreign battlefields and are controlled remotely by pilots.
For example, the MQ-9 Reaper can fly at up to 300 mph, carry an external payload of up to 3,000 pounds, and can remain aloft for up to 14 hours. Although it doesn't carry a flamethrower onboard like the previously aforementioned DJI drone, it can carry a deadly armament of four Hellfire missiles and two GBU-12 Paveway II laser-guided bombs.