Using a 230-grain projectile at 850 feet per second from the muzzle, it’d penetrate through that 9″ torso and when it overpenetrated it’d still be going 498 feet per second. That would give it enough energy to be able to penetrate over 16″ of ballistic gel, definitely capable of a fatal hit. But let’s put it in perspective — let’s say that the .45 ACP FMJ penetrated through the 9″-thick attacker, and continued on to hit a bystander — at 498 feet per second, it’d have enough energy to easily pass completely through 9″ of bystander, and still be going at 252 feet per second! After exiting the bystander, it’d still maintain enough energy to reach almost 8″ deep into ballistic gel — again, far enough to cause serious damage, and depending on where it hits, it may even cause a critical/fatal hit on a person behind the bystander behind the attacker. Yes, one .45 ACP FMJ could pass completely through two people and lodge deeply enough in the third to cause a fatal hit.
Is overpenetration a concern?
Yes, but it’s only a significant concern if you’re foolish enough to load your defensive weapon with FMJ bullets instead of hollowpoints. If hollowpoints are legal for self defense where you live, USE THEM. They’re much more effective wounders, they’re much more likely to stop an attacker, and they vastly minimize any risk of overpenetration. A hollowpoint expands so large that it slows down dramatically while it’s traveling through the attacker’s body; even if it overpenetrates it’ll be going so slowly that it won’t be nearly as dangerous as an FMJ would be. The only time I’d recommend against hollowpoints is when you’re using a tiny caliber (specifically .22LR, .25 ACP, or .32 ACP) where there just isn’t enough energy available to push a hollowpoint deep enough to cause a critical hit — in those cases, you have to go with a non-expanding bullet; wadcutters would be preferred, but use FMJ’s if you can’t get wadcutters.