The Ruger SP-101 is a five- or six-shot double-action revolver introduced in 1989, intended for concealed carry and personal defense. House, in his never-ending quest to find creative ways to use ordinary objects, uses one to shoot a corpse in the head to determine the effects of a bullet on an MRI scan. This incident gives the first indication that Foreman had been infected with the disease which eventually kills the patient (Euphoria (Part 1)). Also, Alice Tanner attempts suicide with one (Unwritten).
House's SP-101 is a double-action-only version with a spurless hammer, meaning it cannot be cocked, while Alice's has a spur and single-action capability, which is why she was able to cock it before her suicide attempt.