I saw a study some years back that correlated calibre and first round knockdown. As one would expect the big bore stuff was in the high ninety percentile range. What suprised me was that smaller bore stuff was good too. As I recall 38s were in the low ninety percentile range. And the lowly 22 was in the mid eighties. That really suprised me.
Most assassins and hit men, whether government-sponsored or free-lance, use the .22 LR.