It may not matter much, but the 40-9 conversion barrel fits the M&P slide and breech face exactly. The factory barrel does not but is close enough. The conversion barrel may be more readily available for purchase as well. The M&P pistol is ideal for conversion since the 40 S&W and 9mm ejector and extractor are the same. Other pistols, like Glock, require a 40-9 conversion barrel to fit the slide.