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Deals damage. The power of this move depends on (user's weight / target's weight), rounded down. Power is equal to 120 if the result is 5 or more, 100 if 4, 80 if 3, 60 if 2, and 40 if 1 or less.
Heavy Slam has little to no competitive use. Unless used by exceptionally heavy Pokemon on extremely light targets, the move will often be outdamaged by more reliable choices, such as Iron Head or Meteor Mash. The exception to this is Choice Band Aggron in RU, where Heavy Slam gets a stronger hit on many of the Pokemon that threaten Aggron more than Iron Head.
Iron Head is a more reliable option if the user is light, and also has a 30% chance to flinch the target.
Heat Crash is a Fire-type equivalent.