I agree with the heavy for caliber bullets like the 9mm 147-gr. providing needed extra penetration through extremities or bones before reaching the vitals. Almost all testing through various media shows a pattern of heavier bullets outperforming their lightweight counterparts across all caliber lines. While middleweight bonded bullets are coming around, heavyweight bonded projectiles still (usually) outperform them.
The FBI uses common sense when it says that 62% of the time in OIC's, a bullet must pass through an arm / leg, or penetrate at an odd angle before reaching the vitals. This is why their minimum standard of 12" is in effect. They also teach to aim a little higher than many PD's (used to) do because there are more significant vital areas between the armpits and above than below that area. More than 1/2 of OIC's are at odd angles where the bullet travels a longer distance than civilians imagine.
When someone's life is on the line, immediately stopping the threat is paramount! BG's usually expire more quickly with higher center-mass hits than the older adage of around the bottom of the ribs / sternum (like the old-style targets encouraged you to aim for). Notice that in the past decade, range targets have raised their center ring about 3" higher than it used to be.
A couple of bullets placed dead-center (or a tad to the right) between the armpits can be your best friend when your life is on the line. Self defense priorities should include:
1. Shot Placement
2. Bullet Construction / Weight
3. Caliber
