So how was it that Richard Davis, president of Second Chance body armor, would shoot himself in the chest while wearing his body armor, as part of his sales pitch?
Ballistic info on a common pistol round, the .45 ACP
Standard .45 ACP ammo has around 400 foot pounds of energy at the muzzle. That's enough to move a four hundred pound object a foot. So if the guy weighs 175 pounds, he's going to get blown backwards at least 2.2 feet. I don't know about you, but that would knock most people cleanly to the ground with no fuss. His demonstration is either rigged, or he is an unnaturally strong or large person. I'd betting more that his demonstration is rigged.
Wrong kind of pound. You are better off thinking in terms of momentumn - lots of the energy of the bullet will go into deforming the bullet and vest.
Momentum-wise, looks like such a bullet would impart a velocity of ~6cm/s to a 175 lb guy. I'd guess it'd look like no more than a slight backward stumble if the guy were prapared for it.
If he holds the gun himself, it'd cancel of course.