I too was just looking at buying the M&P 9mm and then started browsing the forum and read about the .40 conversion to 9mm. So I went to the local gun store and rented the .40 to see how it felt... WOW! It was amazingly accurate and softer than my friend's .40 glock we had with us. So I have decided that's the route I'm going to go, buy the M&P .40 and the aftermarket barrel with 9mm mags.. but the one thing I haven't seen mentioned, or I missed it completely... do you just use the same spring and guiderod that comes in the .40? Is there any difference between the springs in the calibers? From what I'm reading, you are all only buying the 9mm barrel and mags, correct? Thanks for any comments on this.