Honestly AC/DC always felt close enough to metal for me. But I gauge this stuff by whether or not I saw them in Headbangers Ball.
Anyway we are just waiting on
@Crocker and
@Wiseman
I think, for me, it's a question of tempo, vibe and drums. AC DC is a rock n roll happy vibe, the drums are slow and all basic 4x4 tempo. They never play that fast , the drums are not being hit aggressively , etc...now obviously there's exceptions to every genre, and a lot of artists blend genres. But I just don't get metal vibes whatsoever from them. You can look at older bands like Sabbath and Ozzy, the vibe is clearly different than a party atmosphere. The lyrics also play a big part. Metal tends to be much darker, brooding and hits a lot of uneasy subjects. AC DC talks about rocking and fucking bitches for the most part. typical rock.