Ork weapons unreliable outside ork hands

Simply put, Ork weapons should not work. Poorly
engineered, constructed of second rate, hastily scavenged
materials, and built with no care whatsoever for user safety,
they should jam or misfire every time. In fact, while certainly
not paragons of reliability, they work well enough—when
wielded by Orks. For millennia, Tech-Priests have been
aware that Orkish weapons work far less reliably (if at all)
when operated by non-Orks. Various theories have been
advanced to explain this, including the suggestion that Orks
somehow exude a low level psychic effect that renders their
weaponry usable. This view is controversial, though, as no
concrete evidence of the existence of such a phenomenon
has been found.
All normal Ork ranged weapons (and some melee ones
as well) have the Unreliable quality. When such a weapon is
used by an Ork, though, it loses this quality.

- Dark Heresy 2E: Enemies Without, Page 86