Let's see, 53 ft-lbs. A 4 oz. baseball at 80 mph, fairly typical major league fastball, is enough to reliably kill people. Gee, why do they bother letting the batter walk to first base in the event of a wild ball? He should be dead, instead!
Can't find a citation for 53 ft-lbs, other than a couple very vague references, so I'll have to let that one go (though I'm positive I've read some stuff dating between WWI and II that referenced about 65 ft-lbs).
However, the 12"-18" standard was actually arrived at with a number of doctors, mostly Dr. Fackler, based on what the minimum is to reliably reach a vital organ from any shooting angle which could occur in a confrontation.
And the "experience," which I guess you believe is a fairy tail, was 2 FBI agents dying because a guy who had been shot from the side, through his arm, as he was crawling out of a car, took a lot longer to die than he would've if the bullet had penetrated a couple inches more.