Originally Posted By: 901Memphis
Think about if you shot at someone's chest and their arm was in the way of the shot. You have two sides of arm to go through and the chest itself.
If the person were just wearing two shirts that would be 6 layers of clothing to penetrate. Everything gets multiplied by 3 in that situation, so 4 layers of denim is a good test protocol for worst case heavy clothing scenarios.
If you have to shoot through someones arm before the bullet enters their chest cavity, you are not going to care about perfect expansion, with a perfect butterfly of a bullet. You will want penetration. One aspect of penetration that is never accounted for in ballistic gel block testing, is the skin. Skin is very tough, and acts quite different than gel. On an arm hit scenario, the bullet has to enter the arm, exit the arm, and enter the chest cavity, thus passing through the skin at least three times. Another reason why you want a bullet that is a known good perpetrator. Gold dot for example.