Understand the differences in many popular calibers.

 Left to right:  .22 LR, .25 ACP, .32 ACP, .380 Auto, 9mm, .357 SIG, .38 SPL, .357 Mag, .40 S&W, .45 ACP, and .223.

Kinetic energy is a measurement of the potential energy transferred into a target.  Many things can effect the kinetic energy such as distance to a target.  As the distance increases, the velocity at impact is lowered so the transferred energy is reduced.  If a bullet passes completely through the target as is often the case with FMJ ammo, not all of the energy is transferred as the bullet continues its flight.  Still, a comparison of typical kinetic energies is a good judge of the stopping power of a particular caliber.  As you can see by the inclusion of the .223, virtually all rifle rounds have more energy than even the most powerful handguns.