The "Heavy Metal vs. Hard Rock" debate has now raged for decades -- possibly even since Biblical times -- and for excellent scientific reasons. I'm no chemist -- I had to struggle for mere B grades back in high school, which was more than a few years ago -- but it doesn't take much Internet perusal to establish that the metals with the heaviest atomic weights (ununoctium, ununseptium, livermorium, ununpentium... not exactly everyday household substances, I don't think) do not necessarily correspond with minerals with the highest Mohs hardness ratings (carbon in diamond form, boron, chromium, tungsten... forgive me if I missed one or two, I'm an amateur at this). Once in a while, you apparently get an element that's both hard and heavy at the same time, and frequently (see: osmium, rhenium) these wind up being the titles of Parliament albums. Anyway, the point is that heaviness and hardness are two different variables, got it?
Or are they? People of post- Maiden / Priest vintage (many of them old themselves by now) never believe this, but back in the '70s, the genre descriptions "hard rock" and "heavy metal" were almost entirely interchangeable. The discography at the end of Lester Bangs' late-'70s heavy metal essay in The Rolling Stone Illustrated History of Rock & Roll includes albums by not only Black Sabbath, Deep Purple and Uriah Heep, but also Black Oak Arkansas, Cactus, Grand Funk Railroad, Mountain, Sweet and ZZ Top. In other words, back then, knowing how to boogie (or to bubble) did not ding one's metal credentials.
I'm not exactly sure when the branches first split off, but I do vaguely remember an early-'80s Village Voice piece (on Kix and Billy Squier, maybe?) that distinguished between, get this, "heavy rock" and "hard metal," which three decades later I'm still not sure I can wrap my head around. In 1985, I myself wrote this, in the book Rock & Roll Confidential Report: "Glamming of heavy metal has coincided with a decided glitter influence all over the rock spectrum ... The effect on heavy metal is that it's not really 'heavy metal' anymore -- the distinction has always been a hazy one, but it sounds more like 'hard rock' to me. In other words, Ratt's 'Round and Round' has more in common with BTO than Black Sabbath, or with The Stones than Led Zeppelin. It's not a change I complain about much." (What, no Aerosmith?)
Anyway, I bring all this up as a (very) roundabout way of warning purists that some might well consider at least the top four or five albums below more "hard rock" than "heavy metal." What I will say, in my defense, is that they are at the very least heavy hard rock: Much heavier than, I dunno, The Black Crowes or Nickelback or whoever. And it seems to me that, with the recent notoriety given to bands from Mastodon and Graveyard to Royal Thunder and Christian Mistress (not to mention Baroness themselves), genre distinctions are blurring once again. Which is undoubtedly a good thing. But listen and judge for thyself.