Question

Why are Bullet Seed and Arm Thrust usually useless moves for Mega Heracross?

Comments

Arm Thrust is worse than Close Combat because of the lower power (Even after 5 hits).

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You'll probably get a better answer if you ask here: https://www.smogon.com/forums/threads/ask-simple-questions-here-read-original-post-before-posting.3520646/ I'm guessing it's because bullet seed doesn't cover any types that resist bug or fighting, and arm thrust is outclassed by close combat.

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Mega Heracross has Skill Link.

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Skill Link doesn't make the move stronger, it just guarantees 5 hits.  Arm Thrust is 15 per hit, to a max of 75.  Close Combat, in spite of the Defense drop for the user, hits for 120.  Not much of a question there, it's almost double attack power.

Answer 1

Arm Thrust:
Close Combat does more damage. All I can say.
Bullet Seed
A STAB Close Combat hits harder than Bullet Seed in almost all situations. Bullet Seed doesn’t provide meaningful coverage, only hitting Ground and Water harder than Close Combat hits anything, and neither of those are super effective against Heracross. It’s just not worth the moveslot over Pin Missle, Swords Dance, Rock Blast, and Close Combat.