What it does
Sniper does exactly one thing: when the holder lands a critical hit, the damage multiplier goes from the standard ×1.5 to ×2.25.
In practice: a hit that would normally deal 100 damage becomes 225 on a crit instead of 150 — 75 extra damage per crit. The tricky part is crit rate. Base critical hit chance is about 4.2% (1/24). With +1 crit stage (from Scope Lens), it jumps to 12.5%. At +2 it hits 50%, and +3 is guaranteed. Sniper only pays off when you can stack crit stages high enough to make crits happen regularly.
Who runs it
Ariados (5% usage): Ariados doesn't have impressive base stats — Attack 90, Speed 40 — but Sniper gives it a distinct niche. The key is that Ariados learns Cross Poison and Night Slash, both of which have an inherent +1 crit stage. Pair that with Scope Lens (+1 more), and Cross Poison throws crits 50% of the time. When one lands, the ×2.25 multiplier plus Poison-type STAB can punch well above what Ariados' raw Attack stat would suggest.
5% usage is niche but real — there's a small contingent of players who commit to this line.
How to use it
The bread and butter is Scope Lens + Cross Poison (or Night Slash), getting crit rate to 50%. At that rate you're landing a crit every two hits on average — and that ×2.25 swing can drop opponents that Ariados couldn't normally 2HKO.
Speed 40 means Ariados almost always moves last. Don't try to use it as a sweeper. The play is to switch in on something that can't OHKO it, then fish for the crit to dish back meaningful damage. Sticky Web is a useful support option on the same set — slowing the opponent's team lets Ariados survive a bit longer to get its swings in.
Tips & strategy
Crits ignore defensive boosts: Critical hits bypass the target's Defense and Special Defense stat stage increases (their +1 or +2 defense becomes irrelevant when you crit), and they also ignore your own stat drops. If the opponent has Intimidated your Attack, a crit ignores that penalty — which is one indirect reason Sniper sets are sometimes useful against Intimidate-heavy teams.
Cross Poison vs Night Slash: Cross Poison is Poison-type, stronger STAB on neutral targets. Night Slash is Dark-type, covering different matchups (useful against Psychic and Ghost types). Pick based on what you expect to face.
Honest ceiling: The 5% usage is already an overperformance for what Ariados is. It's a luck-dependent win condition — when the crit fires, it surprises the opponent; when it doesn't, Ariados is just a slow, low-Attack Pokémon. Runs it as a rogue pick to catch people off guard rather than a consistent tournament strategy.