There are no "one shot one kill" studies. Too much variable in real-world gun fights, for all the reasons I mentioned. There are some very good studies on terminal ballistics, the FBI's being the baseline, but you can't quantify the effects of one bullet over another merely by caliber when there are so many different loadings, weapons, victims, and physical differences at play. Most any .380 to the heart at close range will kill you, right now. So will a .45, so will a .22lr, so will a .338 Lapua. The differences between LOADINGS, in controlled ballistic testings can be studied and the statistical data on deaths compared, but correlation does not equal causation, but there's no hard data to show that .380 bullets are less lethal than 9mm in real world cases. A shitty 9mm loading is inferior to a good .380, and vice versa. If you want to recommend a larger, better expanding bullet I'll agree with you every time, but you can't paint with a broad brush where calibers are concerned. There are some very good .380 loadings, and they will kill you with one shot, every time if that shot is in the right place. Likewise, there are other shots that aren't perfect that may kill you, may not, and a better loading in another caliber may make the difference. Hydraulic shock, tumbling, fragmentation, and a whole slew of other variables come into play.