Just as a data point, I'm three for three on using readily available semi drop-in Apex barrels to fix inconsistent M&P9s. My sample size is obviously quite small, but I also haven't heard of anyone fitting an SDI barrel and still having accuracy problems. I looked for evidence here, brian enos forum, smith-wesson forum, and mp-pistol forum before going down this road. What few reports are out there are overwhelmingly positive.