Autotomize is applied before other weight-modifying effects, including Heavy Metal. Heavy Metal is the only effect that can cause a Pokémon's weight to exceed 999.9 kg.
This has no effect on the Heavy Ball, which uses the Pokémon's species' weight rather than its current weight.
Heavy Metal
Doubles the weight of the Pokémon with this Ability.
Light Metal
Halves the weight of the Pokémon with this Ability, rounded down to the nearest tenth (e.g., Metang, normally weighing 202.5 kg, will weigh 101.2 kg if it has Light Metal).
Relevant Code Points
Function 0x022468A4 handles damage calculations for Low Kick; this function should be updated to account for Heavy/Light Metal on the defender.
Documentation
https://bulbapedia.bulbagarden.net/wiki/Heavy_Metal_(Ability) https://bulbapedia.bulbagarden.net/wiki/Light_Metal_(Ability)
Shared
Heavy Metal
Light Metal
Relevant Code Points
Function
0x022468A4handles damage calculations for Low Kick; this function should be updated to account for Heavy/Light Metal on the defender.