Here are some pictures of my M&P 9 side by side with my BG 380. Notice picture 4291, the barrels look the same size. Looks to me like there really isn't much of a difference between a 380 and a 9 bullet?
http://s1176.photobucket.com/albums/x321/Maxspeed810/BG vs 9/
http://s1176.photobucket.com/albums/x321/Maxspeed810/BG vs 9/