I've been doing quite a bit of reading on defensive ammo and they always refer to how much penetration a particular round will have. It seems that anything less that 15 inches is not considered good enough. My question is why 15 inches? I'm an average sized male 6 foot ~200 lbs I have a 44 inch chest. My chest is only ~8 inches front to back, 25 inches shoulder to shoulder and 17 inches arm pit to arm pit. Given those numbers even if the bullet were to enter my shoulder it would hit my heart in less than 15 inches in most cases my heart is closer to an average of 6 inches from my skin. With 15 inched of penetration a straight on shot to my chest is going to exit the body which as I understand is not desirable because 1. all of the energy wasn't deposited in the body and 2. an innocent bystander could be hit. So what am I missing?