You are right on it. I am not arguing one way or the other that using a large handgun for protection is a good idea, this is a academic look at the question. A 500 would not be the ideal choice for penetration. Vests are more effective the more surface area of the vest that is contacted and more velocity with a similarly constructed bullet will generally give more penetration. A .50 caliber projectile will contact not twice the surface of a .22 caliber round but closer to 5 times the amount of surface area so the 22 is tougher to stop. The .50 caliber has to defeat 5X the Kevlar compared to a 22. If a big revolver is desired, the 460 is a better choice as it has less surface area and greater velocity while an even better handgun choice would be a 30/30 Contender..