As it turns out I just returned from the range and there was a guy with the 9mm Shield. I have the 40 shield. I let him fire my 40 and I was able to fire his 9mm. IMHO there is a noticable difference. He agreed as well. After shooting, we went out in the parking lot and dis-assembled the weapons. His 9mm barrel fit in my 40, but not vice-versa ... the 40 barrel would not fit properly in the 9mm. I wish I could have fired it with the 9mm barrel but am now very happy I bought the 40.