Is it easy to modify an AR15 to a full Auto? How would you know if someone did this? I think full auto capability isn't that important, I can pull a trigger pretty fast anyway, but I've heard the story about a guy who borrowed his friends AR, which was modified and he went to jail after firing in full auto without meaning to. So, how would you know if it has been modified without test firing it first in case you want to buy a used AR15 via private sale? Would the safety switch move past the fire mark or what?