"in common use for lawful purposes", according to Gura, should be read as "in common use or would be in common use for lawful purposes"
That's the only way we can avoid nonsense arguments such as the circularity argument mentioned above ( because something is illegal (and therefore not in common use) it's therefore not protected).
Assuming Gura is right, then drones may well be protected. They certainly are unusual, and if they're not dangerous they're not useful for self-defense, but would they be in common use in a constitutionally-correct environment?
Probably not at the current time, but eventually robotics will get to the point where having a self-defense robot will be a viable thing. It never needs to sleep, it won't panic, it can't be easily destroyed, you can be moving your family to safety while it fights, etc, etc. Who wouldn't want such a thing protecting their property?
Therefore, the day will come when such things are protected by the 2nd Amendment.