[Targan]

Jeez where do I begin?

Okay, in the case of rifles that fire EXACTLY the same rounds as pistols (in the days of the wild west there were many examples of this) O'Borg is right when he says velocity. Many pistol-type rounds fired from pistols do not travel along a long enough barrel for all the powder to burn and have the gasses expand to their maximum potential, but if you fire them from a pistol with a very long barel or from a carbine or rifle you maximise their potential.

Then there is the matter of energy transferred to the target. A pistol bullet may have less energy than a rifle bullet when it reaches its target (because its case contains much less propellant) but it will usually be quite short and fat with a fairly blunt nose so it is less likely to tear right through the target and even if it does, either way it still transfers a greater proportion of its energy into the target. Rifle bullets are all about range and penetration. Copeab is right about bullets tumbling, and rifle bullets are usually travelling a hell of a lot faster than pistol bullets. So even if a smaller proportion of a rifle bullet's energy is transferred to the target (both because the bullet tears right through the target and because it isn't as fat) it still has such a vast amount of energy to start with that the proportion of energy that is delivered into the target still exceeds a pistol bullet.

Lets look at an example of a pistol type bullet and a rifle type bullet which have the same width, the .22 calibre. A .22 calibre fired from a pistol never even reaches its potential and it never really had much potential to start with. It isn't a very fat round, it isn't very long either so it doesn't have much mass and the amount of powder behind it is, well, pathetic. About the only thing it has going for it is that it is fairly blunt nosed and it will probably stop inside the target so it will deliver all the energy it had upon leaving the barrel (minus a bit of air resistance) when it hits the target. Now 5.56mm is the same as .223 Remington so basically a .22 but it is a much longer projectile and it has WAY more powder behind it so it is moving seriously fast when it hits the target. Whammo. Even if it tears right through the target it has hit with so much more force that alot more energy will be transferred than the pissy little .22, and added to that if it tumbles on the way through it leaves a much bigger wound channel. So hopefully the target is DRT.