Torchwood stun guns worked by "shocking" targets in physical contact with the muzzle in a similar manner to a taser. It had two settings; one for killing and one for stunning.
Gwen Cooper used one of these guns against her boyfriend Rhys, so that she could take him to the Hub without having to convince him to go. (TW: End of Days)
The team used the stun guns when investigating the factory exploiting the Cash Cow. (TW: Meat)
Ianto used it on Billy Davies, a hitman working for the Pharm. (TW: Reset)
On one occasion, Jack Harkness accidentally killed a Histrix when he thought the gun was set to stun. (BBCR: Asylum)