What it does
Bulletproof makes the holder completely immune to ball-type and bomb-type moves — damage and effects both. The blocked list includes: Shadow Ball, Focus Blast, Energy Ball, Sludge Bomb, Aura Sphere, Electro Ball, Mud Bomb, and others in that category.
What it does NOT block: beam moves (Ice Beam, Thunderbolt, Flamethrower) and pulse moves (Water Pulse, Dragon Pulse) go through normally. Bulletproof is specific to balls and bombs, not a blanket special immunity.
Why do balls matter? Shadow Ball and Focus Blast are two of the most widely used special coverage moves in the game. Grass-types get hit by Ghost attacks naturally, and Focus Blast is the go-to coverage for hitting Steel and Dark types despite its shaky accuracy. Bulletproof closes both doors at once.
Who runs it
Chesnaught (98%): Grass/Fighting, and Bulletproof is basically its whole identity in competitive. Ghost-type attackers trying to Shadow Ball through Chesnaught's Grass weakness? Blocked. Special attackers trying to Focus Blast through its Fighting type to dodge resistance? Also blocked. Chesnaught's physical bulk is already solid, and Bulletproof patches the special side such that most coverage-based special attackers simply can't touch it with their usual tools.
Kommo-o (6%): Normally prefers Soundproof or Overcoat — Bulletproof on Kommo-o is an edge case, covering Shadow Ball exposure. The 6% usage tells the whole story: Chesnaught just does this job better.
How to use it
The logic is simple: bring Chesnaught in against special attackers carrying Shadow Ball or Focus Blast coverage, absorb the move for free, and respond. You're trading their item slot's coverage move for essentially nothing on your end.
Spiky Shield pairs naturally here — Chesnaught's signature move blocks all damage and punishes contact moves with 1/8 HP loss. Bulletproof plus Spiky Shield gives you a safe response to a wide range of attackers: immune to most special coverage, punishing physical contact.
Tips & strategy
Beam moves go right through, and that's the thing to watch. Chesnaught has a 4× weakness to Fire because of its Grass/Fighting typing — Flamethrower, Fire Blast, and Heat Wave all hit it cleanly. Bulletproof handles the ball coverage; it does nothing about fire beams.
Aura Sphere is worth a specific mention: it's a never-miss Fighting-type special move, and Bulletproof blanket-blocks it. That matters a lot because Aura Sphere is popular precisely for its accuracy. Against most Chesnaught opponents, that option simply disappears from their usable moves.