Except for the danger to bystanders caused by a pass-through, why is bullet expansion such an important factor to most people?

It's makes a bigger wound channel and causes greater hydrostatic shock, thus being more effective as long as it penetrates deeply enough.

In the simplest terms, bullet expansion makes a bigger hole and results in consequent greater tissue damage than would a bullet that does not increase in diameter.

It is thought that this results in faster incapacitation of a living target than would be obtained with the lesser tissue damage resulting from the smaller hole of a non expanding bullet.

Hydrostatic shock (transfer of energy through the non compressible medium of a liquid resulting in tissue damage remote from the actual wound canal) is a factor in high velocity projectiles such as a high velocity rifle bullet.

Hydrostatic shock probably isn't much of a factor in handgun bullets which travel at very low velocities as compared to rifle projectiles.

At the end of the day, shot placement is certainly more important than bullet expansion. Fringe hits on the BG with an expanding bullet are no substitute for good shot placement.

The object of the game is to transfer energy to the intended target. Bullets which do not expand or deform tend pass through the intended target without transferring much of their kinetic energy, continuing onward with significant remaining energy, thus more likely to cause unintended further damage.

Assuming a pistol bullet of 150 grains in weight, travelling at approx. 1000 FPS, the projectile will have a muzzle energy of around 350-400 Ft. Lbs. Passing through a human body might require less than half of that energy, with the bullet continuing onward with sufficient force to cause serious wounding. But if the bullet can be made so as to stop within the intended target, then all of its energy will be transferred to that target, thus a far more effective result. One way of accomplishing that goal is bullets designed to expand symmetrically, continuing along the intended path while simultaneously increasing in diameter, thus presenting a greater frontal area that serves to both transfer energy more effectively as well as causing the bullet to stop more rapidly (preferably within the intended target).

The same is true for rifle ammunition intended for use on game animals. It has long been known that bullets that provide controlled expansion (without fragmenting or tumbling) are more effective for taking the larger game than non-expanding bullets.

A bullet that does not pass through.........

and delivers ALL its energy inside the body, is a winner, for many reasons.

Other than minimizing the possibility of overpenetration, an expanded bullet will crush more tissue than a non-expanded bullet of the same caliber. Also, an expanded JHP will likely have exposed sharp edges that can cause more trauma than a rounded FMJ; a sharp-edged JHP is more likely to cut open blood vessels than FMJ. Even an unexpanded JHP has the potential to cause more trauma in a wound channel due to a cookie-cutter effect, unlike FMJ which will stretch tissue before breaking through, causing it to close up afterwards.

From what I've researched, hydrostatic shock is essentially non-existent at handgun velocities.

None of this negates the importance of shot placement.

As to whether or not the advantages of JHP are significant enough to choose it over FMJ, that's for you to decide. Personally, I would only use JHP for carry/self defense purposes unless prohibited by law, such as in NJ.

Good assessment.

I would also add that expanding bullets are preferred due to the "cutting" effect of them, or at least with the high quality HPs. With a HST or Ranger, expanding exposes sharp and jagged cutting edges that inflict greater tissue damage.

Not only does it dump it's energy inside a body's cavity but it also minimizes pass through and liability of hitting someone else not intended.

The old Police 158 grain RNL bullet was famous for pass through, no expansion and was NOT a great man stopper - hence the poor performance and reputation it had. Yes, many have died from that round (might have bleed out) however the purpose of shooting a bad guy is to STOP them from causing further harm ASAP, not necessarily kill them. The good expanding bullets along with good penetration stops them faster due to the immediate dump of energy. It also give more surface area to a relatively small diameter bullet.

Expanding bullets also make for more dramatic milk jug busting.

I used to be a believer in bullet expansion until I discovered that the Colt 1851 .36 Navy was considered an effective fighting handgun back in the day.

Too many inconsistencies regarding this subject so I gave up. My 2 inch Mod 60 loaded with standard velocity R-P 158 gr SWC is fine with me.

Excellent answer!