The prop department was located in Tholian space, and the rifle was caught in interphase. The episode "The Tholian Web" is a cover up of what really happened when Kirk went after his favorite gun.
The Desilu prop department might as well have been in Tholian hands, considering the anecdote in INSIDE STAR TREK (Solow, Justman). They took too long to make anything, charged a fortune, and when they finally produced a phaser pistol, Roddenberry examined it and said, "This is uh, uh... unacceptable."