Ork Snipers

Ork snipers are a fan nickname for Vindicare on account of Celestial Lions loosing many personnel, especially their Apothecaries, to snipers that were allegedly orks, despite convincing evidence to the contrary. That said in the Black Library, there are Orkish snipers:

The Ciaphas Cain book, Caves of Ice, has Ork snipers, unless you consider actually being a good shot a requirement for being a sniper. The book Fifteen Hours has a Grot sniper.

On a sidenote, back in 3rd edition, Lootas could loot (amongst other things) Sniper Rifles and use them. They only hit on a 4+ (instead of a 2+ like everyone else using them) to represent the average's Ork poor aim.