I think .41 Action Express had a great deal of potential, but didn't make it. It had a rebated rim matching the 9mm Parabellum, but was otherwise similar to today's .40 S&W. As a result, a simple conversion of a 9mm handgun could be done with a simple barrel swap. I did this to a TZ-75 I had back in the late 1980s, but apparently the barrel wasn't a perfect match for my gun, because I had reliability issues with it. A gunsmith could likely have remedied it, but I just gave up on it, having lost confidence in the idea.
