1.) With modern ,expanding JHP ammo, there is not enough difference in the common defensive calibers to make a difference.
2.) Common calibers are 9mm, .40S&W, .357 Sig, and .45ACP. I suppose 10mm, .357 Magnum, and .38spl could be lumped in as well, but they really arent common anymore.
3.) FBI tests, and real world case studies show that 12" is the golden depth for penetration. Deeper may or may not be more effective, depending on a number of variables, and shallower rarely is as effective. Bullet design plays as much a role in this as power/muzzle energy/velocity, but both of these factors work hand in hand.
4.) This is where it gets less fact based, and more emotion driven. The manufacturers have done a pretty good job of leveling the playing field through bullet design. How each caliber, and its respective load accomplish this is a combination of the above factors, working to achieve the same goal. One must decide which method he prefers.
