I too have read somewhere that penetrating skin, especially exiting skin, can be the equivalent of several inches of penetration in gel. This is interesting, but I think it misses the point.
Gel is not used to give us a correlation to predict real world penetration in living tissue, but rather because accurately predicting real world penetration in tissue is so unrealistically complex as be impractical. Gel is used because it provides a homogeneous, calibrated medium that allows for repeatable, scientifically valid results. Scientific method requires there to be only one control in an experiment. The control is the thing that changes. Using meat targets or other complicated mediums means that any difference in our results can be due to either a difference in our testing medium or the ammunition, thereby making the results unrepeatable and scientifically invalid. Using a simple homogeneous substance like ballistics gel gives us something we can calibrate and means that any difference in our results can only be due to the control, the ammunition being tested, thereby making the results repeatable and valid. The 12 to 18 inch standard is derived to give us a relative comparison. We know from actual shootings that bullets capable of penetrating at least 12 inches of gel will likely penetrate far enough in actual tissue to reach vital structures from most angles, and those penetrating less than 18 inches will likely provide a responsible level of penetration not likely to "over penetrate" an assailant with enough remaining velocity and momentum to be dangerous to innocent bystanders. But living bodies are too complicated for us to have an equation whereby we can accurately predict how far a given projectile will penetrate in living tissue given "x" amount of penetration in gel. We have guidelines, not rules.
One such guideline is that bullets producing at least 25 gr x fps of momentum are less likely to be deflected by bones or changes in density. We know that bullets need an impact velocity of about 2000 fps or more to show a temporary stretch cavity, and so generally speaking handguns are not going to use energy very efficiently, and are only going to show damage done by the permanent crush cavity created by tissue physically displaced by the actual projectile. Exceptions may exist to all of these guidelines, so we consider them more generalizations than set in stone rules, but generally speaking, momentum is more important in handguns than energy. Energy favors velocity because velocity is squared in the calculation. So lighter faster bullets will usually have more energy and expansion, but less penetration. Heavier for caliber bullets have higher sectional densities and more momentum, and typically exhibit less expansion and more penetration. If you are worried about deflection or lack of penetration, it is generally better to give up some energy and go with a heavier for caliber, slower projectile.
