Given equal expansion? Good or bad?  Go.

Apparently can't add a poll in Tech, so pole fail.

Multiple answers enabled, I think.

There is a reason the standard is 12-18 inches. Good enough is,,,,,,,,,,,,,,,,good enough.

Cheers

If good enough was good enough, we'd all be hunting 180 lb trophy whitetails w/ 9x19mm & .45" ACP pistols.  And yet...

I have.

Im comfortable carrying fmj. The difference is very small. I do carry HST though

Shoot enough meat/bone/skin and you'll see that even the same bullets fired from the same gun at the same distances won't always do the same thing.

As we used to say in the Army, "Overkill works."

My 1911's have FMJ in them but I use hollow points in the minor calibers (9MM and .40 S&W).  Fast hollow points.  And they carry about 2X the rounds of the 1911s, so more chances to make holes.

Unlike 'Ballistic Gelatin' humans are far from uniform.

Besides differences between muscle and fat we have those annoying bones inside [let alone cartilage of varying calcification levels].

I saw a rather obese guy at the supermarket the other day.
It would likely have taken more than 18 inches to get into his chest cavity.

Of course the biggest thing he could probbaly do is fall on you.

Some of your answers are not appropriate.  Mentioning FMJ and "expansion the same" makes no sense (ie. the answer makes it sound like FMJ may as good as an HP, but only if the HP fialed to expand).
"You can't to 18" if it expands" sure big expansion usually means less penetration, but your question isn't about that, per the question both expand the same.

If anything, I would say the one that overpenetrates may be a bad bullet design as it should invest the energy into expanding further.  If the third choice was one that penetrates 13" but expands more, that's the bullet I would chose.

Getting 12-18" penetration is the goal (with more/less not being good).  Out of the bullets that penetrate 12-18" I would pick the one that expands the most (regardless of if it penetrates 12" or 18").



Per OP, if two bullets expand the same, and one falls into the 12-18" range and the other more than 18", I would chose the 12-18" one.

Why?  Same caliber, same expansion, and one bullet has more penetration than the other.  Yet you choose the bullet w/ less penetration?

(Some of my answers not being appropriate is a regular problem, my GF tells me.)  

Why?  People aren't made of ballistics gel, they have harder bits mixed in, wear cloths, carry things...

Sufficient penetration trumps expansion. We can argue how much penetration is "sufficient", but I carry Hornady's std pressure 135gr Critical Duty primarily because it offers deeper penetration at the cost of modest expansion. IIRC, trying to penetrate the skin while exiting the body is roughly equivalent to another 4" of ballistic gel so I'm not too concerned w/overpenetration.
Tomac

Penetration in ballistic gel is NOT a 1 to 1 ratio for penetration in a human body. It is just a consistent medium to test bullet performance. They had to come up with a consistent medium that will produce repeatable results since human bodies aren't available for testing on each and every bullet that comes out.

To put it into perspective, a BB is supposed to penetrate about 4 inches in the gel to calibrate it for consistency. A BB fired into a human body will barely break the skin. You can tear off a chunk of gelatin with your fingers but you cannot tear off a chunk of human flesh from a person with the same amount of force.

Bones and skin are already taken into account. Scientist who have come up with the penetration numbers in gel have determined that if a round penetrates between 12 to 18 inches in properly calibrated ballistic gel, it will penetrate a human body through bones and skin at all angles fired to reach vitals. Most vitals are only a few inches from the surface in an average human body.

Why 12 inches is Important

exactly, well said

I don't think you gain anything past 18".
If anything the 18+" is probably overpowered, would have more recoil etc.
It could also mean that the 18+" bullet is just taking a lot longer to expand (if energy is the same).  If a bullet penetrates 2" and then expands to 0.75" and pushes that 0.75" object 12", I think that would be better than one that makes a 9mm hole for 13" and then gradually expands to 0.75 (when it may have already exited the body).

Those are about the only 2 reasons I can think of that you would have same expansion with different penetration.

I'm not overly concerned with collateral damage with overpenetration (in a real world shoot out I think most of the shots miss, even for us internet commandos) but it is something to consider.

FINALLY: The FBI has a LOT more experience with shoot outs than I do, and has done a LOT of research.  If they recommend 18" as a max (and 12" as a min) I would be foolish to think that I know better.

Handguns are generally well below rifle in the damage spectrum.

The gelatin model is being way overthought here.

More places for blood to exit the target are better.

Exsanguination (the fancy name for bleeding out) works.

It is what makes a knife dangerous.