Not much, or, i would have replied; "you must have a .45", because i assure you he was a .45 or nothing kinda guy. Most probable he would have ragged on the .40 cal as well. But to compare 9mm to a BB is quite interesting and probably a kindly joke or a case of total lack of knowledge of ballistics. By far 9mm is the most popular pistol caliber in the world and with premium hollowpoints and proper shot placement it does the job well and is what I carry. A .45 will give you about 20-30% larger wound channel given the same high end hollow point but if you place either in a vital area the bad guy is not going to have a good day and won't know the difference.