Originally Posted by
okie
.32 is just on the very, very edge of the bare minimum horsepower if you get my meaning. It doesn't leave any margin for error. Big dude wearing thick clothes, some light barrier like an interior door or drywall, a hand or arm covering the target, oblique angles, etc. will all pretty much defeat .32 ACP. Even in a straight on, unobstructed gimme scenario, it might fail to penetrate or glance off a rib or something. And that's all in FMJ form. Don't even think about carrying .32 hollow points.
The thing to keep in mind is that people don't just stand there and let you shoot them, and human bodies aren't made out of ballistic gel. They're muscle and bone, and a whole bunch of sinews and intersecting tissue planes. And when confronted with an armed assailant, they reflexively turn their shoulder towards the threat and cover their vitals with their weak hand while shooting back with the strong hand, which just so happens to also be in front of their heart. And naturally they try to duck behind anything available.
9mm only overpenetrates if you're assuming a target standing square to you with their arms down, and no barriers. If assuming all of the above, it's pretty much the bare minimum of what's required to be reliable in the real world.