It all comes down to bullet placement, and to a lesser degree penetration.
A bullet that won't penetrate far enough to reach a vital organ is useless no matter how well the shot is placed, but few cartridges are that weak.
Facial bones and teeth have been known to deflect or even stop a bullet, but thats a rare occurance.
The .38 Special has more than enough penetration to reach vital organs, but even a heart shot with a main battle rifle cartridge is not always fatal. A local man was hit twice in the heart by a 7.62 machinegun while manning a door gun of a helicopter. Both bullets ended up in the same ventrical. long range was a factor there.