S4·E4 Kick the Ballistics

While investigating the mysterious shooting death of a young woman, Castle and Beckett face a cop's worst nightmare when they discover that the gun used in the killing is Det. Ryan's old service weapon - the one that was stolen by the serial killer, 3XK. Now with the clock ticking, the team must work to solve the murder, track down 3XK and retrieve Ryan's gun before the killer strikes again.