If you’ve read some of the recent articles on ballistics, especially the comments on this one, you might logically deduce that for any gun the best bullet is the lightest you can find. After all, lighter bullets produce less recoil and more muzzle energy. How can you lose?
It turns out a number of companies have come and gone trying to exploit this argument to sell extremely light, fast bullets, especially for handguns. Liberty Ammunition is the latest on the scene. A detailed critique of previous ventures that includes extensive explanation of the problem with ultralight bullets is archived here.
The short answer is that very light bullets are very bad for defensive use because they lack penetration, and companies that sell them for that purpose are guilty of misleading advertising. Yes, at short ranges they “dump” more kinetic energy into targets, but that energy does not create the deep wound channels experts know are necessary to physiologically stop aggressive animals (including people) in typical shooting scenarios. That’s the end of the argument as far as consumers are concerned.
I will make two other esoteric observations: First is that on the low end ballistic efficiency decreases with bullet weight. The second is that sectional density decreases with weight, which means that ultralight bullets lose speed (and energy) faster and are more susceptible to deflection in flight.
So ultralights suffer impairments at every stage of ballistic consideration: internal, external, and terminal. At short range their extraordinarily high velocity does enhance penetration through some materials, but that does not mitigate their drawbacks. If you have a specific scenario that requires penetration you should get a rifle and load suited to it. Never load your defensive handgun with ultralight bullets!