Actually the kinetic energy of a moving projectile at a contstant speed is:
KE = Kinetic Energy
M = Mass
V = Velocity
KE = .5 x [M x (V x V)]
This is usually expressed in foot pounds of energy in the English system or Joules in the metric system. In ranking muzzle energy of cartridges for an "Average" load, they rank, lowest to highest, as follows:
.223, 7.62x39, 7.62x51
As for penetration, I recommend reading the book, "The Ultimate Sniper." This book has a section where they test real-world penetration scenarios for multiple loads and calibers. Most of the time, a .308 comes out on top, but not always!
Hope this helps.
In a previous thread, someone posted a link to a site that allows you to punch in bullet weight and velocity and it will calculate muzzle energy for you.