With the go-ahead from Fitz, Agent Grant Ward used this rifle to tranquilize the unstable Mike Peterson. According to Peterson in "The Bridge", a side effect of the shot was to stabilize his metabolism, preventing explosion.
Basically a miracle born of engineering and bio-chemistry, this custom tranquilizer rifle — or as Agent Fitz likes to call it, “The Night-Night gun" — is the perfect tool for stopping someone without killing them. The rifle fires non-lethal bullets with heavy stopping power that break up under the subcutaneous tissue (or skin). From there, the bullets deliver a tiny amount of concentrated dendrotoxin, incapacitating the target long enough for them to be secured and with no harmful side effects. It apparently stabilized the Extremis serum in an unexpected but highly positive side effect.
After proven success of the prototype, the Night-Night Gun was replaced by the ICER at moments when smaller arms were required.