Heavy Balls were introduced way back in Pokémon Gold and Silver. This Poké Ball is meant to catch large and heavy Pokémon as it basically improves the catch rate of Pokémon such as that of Snorlax. However, there is one aspect in Heavy Ball that affects certain Pokémon particularly Beldum, Metagross and the recently introduced Pokémon Sun and Moon Tapus.
Pokémon Sun And Moon: Heavy Ball Mechanics
SciresM decompiled the formula for Heavy Ball and it turns out that contrary to popular beliefs, it's not a multiplier; rather, it adds or subtracts to the Pokémon's base catch rate depending on its weight. Based on his findings, the formula adds 20 (catch rate) if the Pokémon weighs over 100 Kg. However, it also subtracts 20 if the Pokémon's weight is below 100 Kg.
This mechanic affects Beldum and Tapus directly for the fact that these Pokémon all weighs less than100 Kg. Since they have a base catch rate of three, after subtraction, the remaining value would sit at -17 under the effect of Heavy Ball. The game's system basically brings all negative values to 0 consequently making all Beldum and Tapus catch rate to nil once Heavy Ball is used.
As stated in the Pokémon subreddit, when it comes to Beldum from previous generations, only HG/SS has a Johto Balls available. What's more, Beldum in those titles is only obtainable in the new Safari Zone, which means that it can only be caught using the Safari Balls - basically making it exclusive for that generation.
To cut to the chase, this issue makes Beldum and all of the Tapus illegal in Heavy Balls. Based on the new rule of the upcoming competition, illegal Pokémon cannot be used in the tournament. The only way for Beldum and its evolved form, Metagross to be deemed legal into the tournament is via breeding.
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