423
Foom Rule
(slrpnk.net)
Be sure to follow the rule before you head out.
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0- Atk Pidgey Peck vs. Lvl 20 0 HP / 252+ Def Gyarados: 72-85 (97.2 - 114.8%) -- 81.3% chance to OHKO
With neutral natures and no EVs it is a OHKO however.
how did you work all this out??
https://calc.pokemonshowdown.com probably