tim west

I'm trying tor remember a title of an old film(1970's?) set in a cinema, on screen there is a western playing, two cowboys end up drawing guns and shooting at each other, when the lights go up in the cinema, one of the audience is found dead by gunshot wound.
A detective who had also been watching the film quickly gets the doors of the cinema shut and then starts to interview all the people held inside, he decides to rerun the events leading up to the murder and gets a volunteer to sit in the victims chair, the lights go down and the film is rerun, after the shootout on screen the lights go up and lo and behold the volunteer is dead by gunshot!!
It turns out the cowboy on the film is the murderer!
I can remember the details but cannot remember the title of this film , please can someone help?