it is true that the founding fathers could not have foreseen the invention of more modern firearms, let alone the idea of semi and fully automatic firearms. but they did clearly envision that a armed populace was a good thing. they also fairly clearly envisioned it as a equally armed type of thing. what was modern then would be considered antique and primitive just a short century later. but the spirit of the amendment still comes through fairly clearly.

in the case of modern society, fully automatic weapons are heavily regulated and are not, in general, in the hands of the populace. true some can be modified, but to be caught with such a weapon, is going to bar you from legally having any sort of firearm, as it’s a felony.