Data suggests different timings for the introduction of these theories and the conflict remains unsolved. Agenda setting was used in World War but so was magic bullet, so which came first and why?
There are some serious doubts whether there was ever a real 'magic bullet' theory in the first place. It's a bit of a scientific urban legend - it's pretty hard to track any early text that argue that simple. Some say it was more of an invention of later researchers to make their work look more advanced... (and sure: there was propaganda research with pretty limited models of human beings being 'programmed' by media messages). Agenda setting was much later, at least if you think of 'classic' agenda setting research (think McCombs, Donald Shaw, David H. Weaver). Naturally, the basic ideas have been around for much longer, but not in the form of formalized theories and studies. Classic agenda setting theory starts in the 70s, with some foundations in the 60s.
Magic bullet theory started with the alchemists looking for the ultimate magic bullet, the 'philosophers stone' Magic bullet theory today is sustained by the press who are always looking for the miracle cure breakthrough. I agree with Thorsten, the theory in scientific terms is an urban myth. No real scientist would ever think that they can find the ultimate truth or the single answer to any problem.