What it does
Heavy Metal doubles the holder's weight, full stop. For Aggron, that means 360kg becomes 720kg.
The reason this matters: Heavy Slam's power scales with the ratio of the user's weight to the target's weight. Specifically:
- Ratio ≥ 5 (user weighs ≥ 5× the target) → 120 power
- Ratio 4–5 → 100 power
- Ratio 3–4 → 80 power
- Ratio 2–3 → 60 power
- Ratio < 2 → 40 power
At 720kg, Aggron hits the 120-power bracket against anything that weighs under 144kg — which is the vast majority of Pokémon. Steel-type, 120 base power, STAB. That's a strong hit.
Heat Crash works the same way but deals Fire damage, covering Grass and Steel types that resist Heavy Slam.
The downside: the opponent's Low Kick and Grass Knot also scale with weight, in the other direction — heavier target, more damage. At 720kg, both moves hit for their maximum 120 power against Aggron. Grass is already a weakness, and Low Kick at 120 power from a Fighting-type is a problem. Aggron's defense is 180, which helps absorb Low Kick — but its 4× weaknesses to Fighting and Ground mean those coverage moves are legitimately dangerous.
Who runs it
Aggron runs Heavy Metal on about 12% of its sets, specifically when the team is built around Heavy Slam as the primary Steel STAB. The other option, Rock Head, only helps if you're running recoil moves like Double-Edge — if your set doesn't include those, Rock Head does nothing and Heavy Metal is the obvious pick.
How to use it
Straight attacking. Lead or pivot Aggron into matchups where it can swing Heavy Slam immediately — Fairy-types, lighter physical attackers, anything that doesn't resist Steel. Heat Crash fills the secondary slot for Grass-types and anything you want Fire coverage on.
Scouting opposing weights matters. A heavy Pokémon like Snorlax (460kg) brings the ratio down to roughly 1.57, which drops Heavy Slam to 40 power. Against those targets, switch to Fire Punch or Heat Crash — don't waste a Heavy Slam into a heavy Pokémon.
Tips & strategy
The choice between Heavy Metal and Rock Head is simple: are you running Double-Edge? If no, Rock Head is useless on this set and Heavy Metal is the better pick. The 12% Heavy Metal rate reflects sets that commit to the Heavy Slam route.
Know Aggron's weight trouble spots before the match. Anything over 144kg starts reducing Heavy Slam's power. Anything over 360kg brings the ratio under 2:1, dropping it to 60 power. Check the opponent's roster for heavy Pokémon and plan accordingly.
Mega Aggron (via Omni Ring) is a totally different Pokémon — pure Steel type with Filter reducing super-effective damage by 25% (a 2× hit lands as 1.5×, a 4× hit lands as 3×). Heavy Metal doesn't apply to the Mega form because the ability gets replaced. If you're going Mega, that's a different set entirely.