I would define it as any weapon which would be overkill (no pun intended) if you were to get them for any purpose aside from destruction, such as if you excused its possession on account of self-defense. One could say a regular handgun suffices as a tool of self-defense, you could take one shot and prevent the advancements of a perpetrator. To get something like an automatic gun of war or a flamethrower, supposedly for purposes of protection, is the self-defense equivalent of parents catching a small child eating all the cake while nobody is looking only to excuse themselves with “but I was hungry”.