A rather contentious issue in the area of self defense is that of proper bullet weight/velocity – I would guess that this is second only to the “which caliber” or “which gun” question.  On one side is the idea that a lighter weight but faster moving bullet is superior, while others contend that a slower moving but heaver bullet is the better option.  This article addresses my thoughts on the issue:

Why bullet weight/velocity trade off occurs
First a quick and simple primer on the physics involved: Firearms burn gun powder to accelerate projectile(s) down the barrel and on toward the target.  The amount of gun powder that can be placed in a given cartridge is limited, primarily by what the gun can handle without experiencing a catastrophic failure due to excessive pressure as the powder burns.  This means that the propulsive force is limited, and the primary factors for bullet performance become bullet weight and bullet speed.  Physics demands that a reduction in bullet weight will result in that bullet being accelerated to higher speeds, while an increase in bullet weight will result in that bullet being accelerated to lower speeds.  In sum, you can either have a bullet that is heaver but slower, or lighter but faster.  Finally, a bullet that travels faster will fall a shorter distance toward the ground on its way to its target, making aiming a bit easier at longer ranges.

The case for a lighter weight but faster bullet
Those who advocate for a lighter but faster bullet will point to the fact that the formula for kinetic energy is 0.5mv^2 (1/2 of the mass x the square of the velocity); meaning that a lighter bullet at a higher speed will carry more kinetic energy than a heavier bullet at lower speeds.  Some will argue that “hydrostatic shock” or “energy transfer” from a high speed bullet will incapacitate the target, independent of the tissue and organs actually struck by the bullet – however this argument seems to lack any basis in fact, at least at the speeds reached by handguns, shotguns, and many rifles.  Another argument in favor of a lighter but faster bullet is the likelihood of more effective and reliable performance of a hollow point bullet when it is traveling faster – this is because a hollow point bullet that is moving too slowly may not reliably expand.  Finally, a lighter bullet can offer reduced recoil without sacrificing kinetic energy levels, which can matter for those physically unable or unwilling to tolerate much recoil.

The case for a heavier but slower bullet
Those who advocate for heavier but slower bullets will argue that bullets work for self defense by penetrating the attacker’s body and physically disrupting vital organs and tissues, not by transferring kinetic energy.  They will also point out that a bullet that is way too light can fail to penetrate deeply enough to reach vital organs, making that bullet unable to stop an attacker.  Finally, advocates of heavier & slower bullets will say that modern hollow points reliably expand at a wide range of velocities, including lower speeds.

My Opinion
I prefer bullets that are slightly lighter than normal, traveling at a slightly faster than normal speed, and believe that this combination will offer the best overall performance.  Note that I say “slightly” lighter and “slightly” faster, as when taken to extremes, a very fast but very light bullet is not very effective, just as a very heavy and very slow bullet is not very effective.  In the self defense scenario, a bullet’s job is to penetrate the attacker and disrupt vital organs in order to stop the attacker from continuing their attack.  A slightly lighter bullet will provide the required penetration, and help ensure good expansion of hollow point bullets, while at the same time providing less recoil (which helps allow accurate follow-up shots).

An example of what I find to be the ideal trade off between velocity and weight
In my favorite pistol caliber, .40 S&W, I prefer a 140 grain homogeneous copper hollow point bullet driven to about 1200 – 1300 feet per second (depending upon barrel length), as offered by Cor-Bon.  Weighing in at 140 grains, this bullet is on the lighter side of most .40 S&W offerings, but will reliably penetrate more than 10 inches of ballistic gel.  Recoil is softer than 155 or 165 grain rounds, the bullet retains about 100% of its weight, and reliably expands to .64 caliber while penetrating to about 12 inches.

As with my artiles addressing “which caliber for self defense” or “which gun for home defense“, I’m sure that opinions here will vary :)