Expansion vs Penetration

[engineglue]

Accurately placed expansion bullets plus good penetration without over penetration would be ideal. I'm sure that's obvious but a difficult balance. In practical use, this is what usually happens;

(I'll define "vital" as a large artery, heart, spine or brain.)

1. The bullet expands and comes in contact with a vital.
2. The bullet expands and doesn't penetrate deep enough to come in contact with a vital.
3. The bullet doesn't expand and misses a vital due to the bullets smaller diameter.
4. The bullet doesn't expand and hits a vital due to accurate shot placement.
5. The bullet over penetrates and potentially causes collateral damage.

To consistently achieve the above five possible outcomes, we would need the following:

1. For the first case; A mid to high powered cartridge (1000fps+), with excellent expansion design.
2. For the second case; A low to mid powered cartridge (1000fps-), with excellent expansion design.
3. For the third case; A mid powered cartridge (1000fps).
4. For the fourth case; The same as the third case.
5. For the fifth case; A mid to high powered cartridge (1000fps+).

A mid powered hollow point could teeter between no expansion and under-penetration.

A mid powered full metal jacket could teeter between missed vitals and over-penetration.

It seems that a non-expanding bullet in a high powered cartridge would be the most reliable. Relying only on the shooters ability.

Note:
1. FPS alone does not tell you how much damage will be caused but it can tell you if the bullet will expand.
2. A 124 grain FMJ 9mm fired from a barrel which delivers 1,000+ fps should provide adequate penetration to hit a vital with the chance of over penetration.

[biggfoot44]

Ooookay. Velocity is the only factor mentioned as to "power level" . Not that any of the following is the be all and end all , but bullet wt , and dia/ sectional density would flesh out a more complete model.

What is "excellent expansion" , "overpenetration" , and "underpenetration" ? I have my views , but knowledegable individuals have been hotly debating all those , and a lot more since at least the era that Lee Jaruas sold the first commercially succuesful line of reliably expanding ammo..

(FWIW I am of the thought that there are more than one aproach to ammo design that works well in the real world , and results are more important than theoretical checklist .)

Give any of the stated reasoning , I'm scratching my head how the leap was made that the prefered 9mm ammo was 124fmj @ a mild vel ?

As controversial as 9mm ammo choices remain to this day , it would be a safe prediction that there would be a strong concensus than any of the top 10 selling major brand 124-127 jhp loads would be much better choice than fmj.

[iraiam]

I only use FMJ for target shooting, for SD I always carry HP ammunition.

In 9mm, I have been shooting 147 grain bullets more than 1000 fps for almost 20 years, lead, FMJ and HP. In the past many of them were +P loads, but now days with our current selection of powders, it's pretty easy to get a 147 grain bullet above 1000 fps.

The possibility of over penetration should be assumed on ANY ammunition, I am a firm believer that thought should be put into what is behind the intended target, even in a SD situation. Much of my training advocates this as well.

[skoro]

In many, if not most, cases, penetration > expansion.

[engineglue]

I saw an xray of a man who had been shot with two 45 caliber bullets. The bullets were hollow point and they had expanded. One of the bullets penetrated a lung and the other bullet did not make it past the sternum. I'm not convinced by the results of ballistic gelatin tests.

[buck460XVR]

Originally posted by engineglue:

Accurately placed expansion bullets plus good penetration without over penetration would be ideal. I'm sure that's obvious but a difficult balance.

This has been the desire of bullet makers and handgun shooters since the advent of jacketed bullets. Problem is, that it is impossible to manufacture a bullet that will do all of that in every scenario, regardless of shot placement, body type, clothing worn and the firearm it is shot from. Regardless of what bullet one uses, there will always be some kind of compromise.


When it comes to SD/HD, accuracy and reliability trumps penetration and expansion everytime. Most times the size of the hole in the BGs heart/brain will have little to do with how fast he dies. How does a hole made by a .45 FMJ in a person's lung any different than one made by a nicely expanding 9mm? Expecting a expanding bullet to compensate for a near miss to a vital organ is foolishness. That's not what they're about. Over-penetration is more a factor of caliber than bullet type, and missing the target all together means one has a better chance of harming someone else beside the BG they're shootin' at.

[JD0x0]

I saw an xray of a man who had been shot with two 45 caliber bullets. The bullets were hollow point and they had expanded. One of the bullets penetrated a lung and the other bullet did not make it past the sternum. I'm not convinced by the results of ballistic gelatin tests.

Nor should you be convinced. Ballistics gel is not a human or animal. Ballistics gel only offers a repeatable test in soft tissue in order to compare expansion rates, penetration, etc.
And the example you listed is why I am not convinced that HP's are the end all self defense bullet type. I'd much rather have pass-throughs with little to no expansion than any bullet stopping at the sternum, or not being able to pass though the skull, or another bone, while I am aiming at a vital organ.
I don't care that pass-throughs are "wasted energy" I'd rather have that energy "wasted," than all dumped at once, where it can't even make it through the sternum. Now, if we were talking a high powered rifle, with enough energy to cause hydrostatic shot that will destroy organs, well that's a different story than measly pistol rounds. In that case, having all that energy dumped quickly could be beneficial.

[ClydeFrog]

I've posted remarks in the past about how I think the KE or kinetic energy level of a handgun round is highly under-rated.
A fast, powerful round that feeds well & offers good marksmanship is what's ideal for a duty or protection round(caliber).

The .357sig is a great example. The caliber was R&Ded by SIG Sauer engineers to replicate the .357magnum JHP out of a 2.5" revolver barrel.
The KE level is higher than other defense loads; .380acp, 9x19mm, .40S&W, .38spl.
Some LE officers & armed citizens gripe about the muzzle blast & recoil but those can be addressed with proper training.
The little known .40Super caliber also had a high KE level & would have been a potent defense load but it never really caught on in the USA.

[Frank Ettin]

Originally Posted by ClydeFrog

I've posted remarks in the past about how I think the KE or kinetic energy level of a handgun round is highly under-rated.
A fast, powerful round that feeds well & offers good marksmanship is what's ideal for a duty or protection round(caliber)....

On the other hand, let's consider how shooting someone will actually cause him to stop what he's doing.

1. The goal is to stop the assailant.
   
   
2. There are four ways in which shooting someone stops him:
   
   
   1. psychological -- "I'm shot, it hurts, I don't want to get shot any more."
      
      
   2. massive blood loss depriving the muscles and brain of oxygen and thus significantly impairing their ability to function
      
      
   3. breaking major skeletal support structures
      
      
   4. damaging the central nervous system.
   
   Depending on someone just giving up because he's been shot is iffy. Probably most fights are stopped that way, but some aren't; and there are no guarantees.
   
   Breaking major skeletal structures can quickly impair mobility. But if the assailant has a gun, he can still shoot. And it will take a reasonably powerful round to reliably penetrate and break a large bone, like the pelvis.
   
   Hits to the central nervous system are sure and quick, but the CNS presents a small and uncertain target. And sometimes significant penetration will be needed to reach it.
   
   The most common and sure physiological way in which shooting someone stops him is blood loss -- depriving the brain and muscles of oxygen and nutrients, thus impairing the ability of the brain and muscles to function. Blood loss is facilitated by (1) large holes causing tissue damage; (2) getting the holes in the right places to damage major blood vessels or blood bearing organs; and (3) adequate penetration to get those holes into the blood vessels and organs which are fairly deep in the body. The problem is that blood loss takes time. People have continued to fight effectively when gravely, even mortally, wounded. So things that can speed up blood loss, more holes, bigger holes, better placed holes, etc., help.
   
   So as a rule of thumb --
   
   
   • More holes are better than fewer holes.
     
     
   • Larger holes are better than smaller holes.
     
     
   • Holes in the right places are better than holes in the wrong places.
     
     
   • Holes that are deep enough are better than holes that aren't.
     
     
   • There are no magic bullets.
   
   
3. With regard to the issue of psychological stops see
   
   
   • this study by Greg Ellifritz.
     
     As Ellifritz note in his discussion of his "failure to incapacitate" data (emphasis added):

     Originally Posted by Greg Ellifritz

     
     ...Take a look at two numbers: the percentage of people who did not stop (no matter how many rounds were fired into them) and the one-shot-stop percentage. The lower caliber rounds (.22, .25, .32) had a failure rate that was roughly double that of the higher caliber rounds. The one-shot-stop percentage (where I considered all hits, anywhere on the body) trended generally higher as the round gets more powerful. This tells us a couple of things...
     
     In a certain (fairly high) percentage of shootings, people stop their aggressive actions after being hit with one round regardless of caliber or shot placement. These people are likely NOT physically incapacitated by the bullet. They just don't want to be shot anymore and give up! Call it a psychological stop if you will. Any bullet or caliber combination will likely yield similar results in those cases. And fortunately for us, there are a lot of these "psychological stops" occurring. The problem we have is when we don't get a psychological stop. If our attacker fights through the pain and continues to victimize us, we might want a round that causes the most damage possible. In essence, we are relying on a "physical stop" rather than a "psychological" one. In order to physically force someone to stop their violent actions we need to either hit him in the Central Nervous System (brain or upper spine) or cause enough bleeding that he becomes unconscious. The more powerful rounds look to be better at doing this....

   • Also see the FBI paper entitled "Handgun Wounding Factors and Effectiveness", by Urey W. Patrick. Agent Patrick, for example, notes on page 8:

     ...Psychological factors are probably the most important relative to achieving rapid incapacitation from a gunshot wound to the torso. Awareness of the injury..., fear of injury, fear of death, blood or pain; intimidation by the weapon or the act of being shot; or the simple desire to quit can all lead to rapid incapacitation even from minor wounds. However, psychological factors are also the primary cause of incapacitation failures.
     
     The individual may be unaware of the wound and thus have no stimuli to force a reaction. Strong will, survival instinct, or sheer emotion such as rage or hate can keep a grievously wounded individual fighting....

   
   
4. And for some more insight into wound physiology and "stopping power":
   
   
   • Dr. V. J. M. DiMaio (DiMaio, V. J. M., M. D., Gunshot Wounds, Elsevier Science Publishing Company, 1987, pg. 42, as quoted in In Defense of Self and Others..., Patrick, Urey W. and Hall, John C., Carolina Academic Press, 2010, pg. 83):

     In the case of low velocity missles, e. g., pistol bullets, the bullet produces a direct path of destruction with very little lateral extension within the surrounding tissue. Only a small temporary cavity is produced. To cause significant injuries to a structure, a pistol bullet must strike that structure directly. The amount of kinetic energy lost in the tissue by a pistol bullet is insufficient to cause the remote injuries produced by a high-velocity rifle bullet.

   • And further in In Defense of Self and Others... (pp. 83-84, emphasis in original):

     The tissue disruption caused by a handgun bullet is limited to two mechanisms. The first or crush mechanism is the hole that the bullet makes passing through the tissue. The second or stretch mechanism is the temporary wound cavity formed by the tissue being driven outward in a radial direction away from the path of the bullet. Of the two, the crush mechanism is the only handgun wounding mechanism that damages tissue. To cause significant injuries to a structure within the body using a handgun, the bullet must penetrate the structure.

   • And further in In Defense of Self and Others... (pp. 95-96, emphasis in original):

     Kinetic energy does not wound. Temporary cavity does not wound. The much-discussed "shock" of bullet impact is a fable....The critical element in wounding effectiveness is penetration. The bullet must pass through the large blood-bearing organs and be of sufficient diameter to promote rapid bleeding....Given durable and reliable penetration, the only way to increase bullet effectiveness is to increase the severity of the wound by increasing the size of the hole made by the bullet....

   
   
5. Urey Patrick was in the FBI for some 24 years, 12 of which were in the firearms training unit where he rose to the position of Assistant Unit Chief. John Hall is an attorney who spent 32 years in the FBI, including serving as a firearms instructor and a SWAT team member.
   
   
6. And sometimes a .357 Magnum doesn't work all that well. LAPD Officer Stacy Lim who was shot in the chest with a .357 Magnum and still ran down her attacker, returned fire, killed him, survived, and ultimately was able to return to duty. She was off duty and heading home after a softball game and a brief stop at the station to check her work assignment. According to the article I linked to:

   ... The bullet ravaged her upper body when it nicked the lower portion of her heart, damaged her liver, destroyed her spleen, and exited through the center of her back, still with enough energy to penetrate her vehicle door, where it was later found....

[45_auto]

Originally Posted by clydefrog

I've posted remarks in the past about how I think the KE or kinetic energy level of a handgun round is highly under-rated.

Yeah, that KE number is real useful. If you were trying to stop someone attacking you, would you rather hit them with 400 ft-lb of KE or 1,000 ft-lb of KE?

My Suburban at just over 2 MPH has 1000 ft-lb of kinetic energy. The bullet I shoot out of my 9mm only delivers 400 ft-lb.

My personal bet is that someone hit with the 400 ft-lb of energy described above will stop what they're doing a lot quicker than someone hit with the 1000 ft-lb of energy described above.

[idek]

I normally try to avoid referring to other people's perspectives on debated topics, but since Frank Ettin already broke the ice on Greg Ellifritz, here's an article that's somewhat interesting. Granted, it's more of a caliber caparison than a bullet comparison.
http://www.buckeyefirearms.org/handgun-stopping-power

I don't know that this data is truly unbiased, however. The author says he only included results from criminal altercations or military battle. That right there will change the picture a lot.

Are military combatants, who are trained and mentally prepared for fighting, going to as easy to stop as some petty thief who's breaking into homes looking for electronics? And what are these military combatants probably being shot with? I suspect that 9mm and .45 autos look worse in the results simply because they tend to be used when more determined combatants are involved.

So even though the author says he has no dog in the fight, I don't believe any data, no matter how large the sample, can really paint the whole picture so long as the situations and variables are not controlled and identical.

Nonetheless, it's still interesting to look at, even if the only real conclusion is, as Frank stated, "There are no magic bullets."

[Deaf Smith]

Energy, momentum, bullet construction, bullet area all come into play.

Energy is more a function of velocity than weight. It has more to do with destructive potential.

Momentum is an even function of weight and velocity. It is a indicator of penetration.

Bullet area is a function of size of the projectile and is an indicator of how large a crush cavity you will get.

Bullet construction is a combination of material and design. It can be used for better penetration or enhanced destruction. It's very hard to get a bullet that is constructed to penetrate alot and yet do great damage.

Any ammunition you get is a compromise. You can have high energy and low weight to enhance destructive capabilities or lower energy but more momentum (weight) for more penetration. But with good bullet construction you can get more penetration or destruction.

And of course larger bullet area can be used to get more of each or both.

If you want it all, real good penetration and real good energy and excellent bullet construction you need to go to some high velocity heavy weight big bores like .44 magnum DPX (ouch!) And that load demands a heavy big gun!

Seriously folks, it is mighty hard to get one type of ammo that does it all.

Mighty hard!

Deaf

[2damnold4this]

Frank, it almost seems like this topic might have come up before as prepared as you were with your response.

[Frank Ettin]

Originally Posted by 2damnold4this

Frank, it almost seems like this topic might have come up before as prepared as you were with your response.

At least five times on TFL in less than a year (since September, 2012, to be exact).

• http://thefiringline.com/forums/showthread.php?t=528178
  
  
• http://thefiringline.com/forums/showthread.php?t=524098
  
  
• http://thefiringline.com/forums/showthread.php?t=508466
  
  
• http://thefiringline.com/forums/showthread.php?t=505180
  
  
• http://thefiringline.com/forums/showthread.php?t=501875

[s4s4u]

Shoot fatter bullets, no need for expansion.

[bt380]

Check out the Sept/Oct 2013 magazine "American Handgunner" article by Greg Ellifritz. The credentials for Greg is impressive and his data isn't just from the gels, it's from actual wounds. It basically supports what many have said, shot placement and especially the control of follow up shots count the most. The differences between 380, 9mm, 40 and 45 cal are not so separated as many may think. There were other points interesting in the read. The "chart" will surprise you and may change the thinking of some, especially the stats on the 380. The magazine had the chart but didn't see it on the url. Support the magazine and spend the 6 dollars for it. For quick look, http your way to americanhandgunner dot com/defensive-carry.

[ClydeFrog]

I say high KE rounds like the .357sig or .40S&W or 10mm have value because if you shoot at a human being the shock & impact of the entry wound be higher than a smaller, lower KE level round.
Blood loss, shock and trauma can cause a subject to stop fighting. Keep in mind that drugs or alcohol may also prevent a rapid reaction too.
Even a .44spl, .45LC or .45acp rd may not stop a spun-out, hyper drug addict.


Clyde

[Frank Ettin]

Originally Posted by ClydeFrog

I say high KE rounds like the .357sig or .40S&W or 10mm have value because if you shoot at a human being the shock & impact of the entry wound be higher than a smaller, lower KE level round....

And exactly what evidence do you have to support that claim? It might be your opinion, but it doesn't mean anything unless you can back it up with evidence.

See post 9:

And further in In Defense of Self and Others... (pp. 95-96, emphasis in original):

[Tomac]

All handguns are relatively poor "stoppers" regardless of caliber or bullet used. Shot placement and sufficient penetration are paramount, all else is secondary.
Expanding bullets? If you carry a load that's capable of sufficient penetration *and* expansion then the additional damage can only increase the bleedout rate that leads to incapacitation due to drop in blood pressure and oxygen deprivation to the brain. In general this is a good thing.
However, there's no scientific way to prove if this expanded bullet will increase the bleedout rate enough (seconds? minutes? hours?) to alter the outcome of the encounter (doesn't do you much good if he passes out from bloodloss in the ambulance from JHP's instead of the ER from FMJ *after* planting daisies in your hair).
I follow the conventional wisdom of carrying the most powerful caliber I can shoot both accurately and quickly w/quality expanding bullets (except for .380's where it's FMJ only as sufficient penetration trumps expansion every time) but don't believe that expanding bullets will significantly increase the chances of my hits ending the fight (YMMV).
Tomac

[orionengnr]

There are a number of false premises in your original post. Start with this one.

FPS alone does not tell you how much damage will be caused but it can tell you if the bullet will expand.

It is clear that your experience and knowledge are limited, or else you have been given a bunch of outdated/bad information.
That, combined with your low post count, leads me to the following advice:

Spend a year here reading a lot and posting a little. The occasional question, yes...but most questions have already been answered many times over (see Frank's post #14) and a search will help with that.

[tetvet68]

Good morning folks,

Just joined TFL and find it very interesting. I'm a long time shooter, hunter, combat vet, hand loader. I've been on other forums and found some of them to be loaded with never-been-in-combat experts who are certain they will react positively in a SD situation with the associated huge adrenalin dump, tunnel vision, etc. Some people out there are really scary and not helping deflect the present anti-gun rhetoric. Some are "trollers" for sure but the internet and Youtube have created a lot of self-proclaimed experts. Some are very credible, some not so much.

That said, the TFL forums I have visited so far are very good, very interesting and there appears to be a fair amount of courtesy for the opinion of others.

In regards to the conversation concerning penetration vs expansion, I am certainly in agreement that one bullet cannot do it all and certain amount of penetration is required to stop a BG. Personally, for me, the bigger the handgun bullet the better and 12" to 15" of penetration with an expanding bullet (Critical Duty, HST, etc) is probably quite acceptable. I'm in favor of the big hole and a big "energy dump". After having been in the jungle, I've seen a person keep going after being hit in the torso. In many case, initial pain was not an issue because the high-power non-expanding rifle bullet zipped right through without the attendant organ shock of an expanding style hunting bullet that dumps a tremendous amount of energy before it exits. As mentioned above, the mere fact of knowing you've been shot is enough to stop many people...call it "psychological shock value". But the body seems to go into almost instant shock, numbing the initial pain, so hitting a vital organ causing massive blood loss (oxygen deprivation to the brain) is important and most vital torso organs are within a few inches of the skin so the bigger the hole, the more chance you have to induce massive blood loss...especially with multiple hits.

I carry a G23 w/night sights and my house gun is a Taurus P1911 with a 4 1/2 lb trigger and night sights. I also have a Taurus 99 9MM and a Ruger MKII with a bull barrel. I practice with all of them with emphasis on the G23 and 1911. I'm not a bullseye shooter but I'm fairly decent at center-mass. I hope I never have to do a Zimmerman but I do practice turning the gun immediately upon clearing the holster and getting the first shot off from slightly above waist level just in case the BG is "right there" because, in my mind, that's the very worst case scenario...other than taking a hit.

Anyway, just wanted to say "Hi" and my initial impression is this will be a fun informative forum. Jim

[RBid]

More holes are better than fewer holes.


Larger holes are better than smaller holes.


Holes in the right places are better than holes in the wrong places.


Holes that are deep enough are better than holes that aren't.


There are no magic bullets.

I quoted the above, because of consistency. When we spend enough time looking at this stuff, it is inevitable that we'll run across people who bash Ellifritz, or Marshall & Sanow, or Doc Roberts, or Fackler, etc. People will point out flawed methodology, insufficient sample size, or any other thing to discredit a particular "expert" whose views they disagree with. You'll hear terms like "gelatin junkie", and run into the "lab science vs street science" debate.

When you step back from all of that, and look for commonality between people with extensive street experience, scientists, enthusiasts, etc, the above points are almost unanimously present. We may agree or disagree on other specifics, but it should be accepted that the statements above are all true.

[Tomac]

Well said, RBid.
Tomac

[idek]

When you... look for commonality between people with extensive street experience, scientists, enthusiasts, etc, the above points are almost unanimously present. We may agree or disagree on other specifics, but it should be accepted that the statements above are all true.

agreed

...it is inevitable that we'll run across people who bash Ellifritz, or Marshall & Sanow, or Doc Roberts, or Fackler, etc. People will point out flawed methodology, insufficient sample size, or any other thing to discredit a particular "expert" whose views they disagree with. You'll hear terms like "gelatin junkie", and run into the "lab science vs street science" debate.

I realize the overall post above is mostly about agreeing on some generally accepted "Truths" and cutting out a lot of the clutter. That is valid, but the phrasing in some parts makes it sound like it's bad to question the findings of these researchers... that people are trying to "bash" researchers or shoot holes in their work simply to satisfy our own egos or something.

In the Ellifritz link I posted, his results showed 61% one-shot incapacitation with .22lr, 72% with .32 caliber cartridges, 62% with .380 ACP.

Meanwhile, 9mm Luger and .45 ACP results show 47% and 51% one-shot incapacitation respectively.

According to this particular study, even the "larger holes are better than smaller holes" statement would go out the window.

Something seems wrong when .32 ACP/.32 Long looks like the clear winner in a comparison of handgun cartridges. If that means I'm "bashing" Ellifrizt's work, then I guess I'm bashing it.

[Frank Ettin]

Originally Posted by idek

...In the Ellifritz link I posted, his results showed 61% one-shot incapacitation with .22lr, 72% with .32 caliber cartridges, 62% with .380 ACP.

Meanwhile, 9mm Luger and .45 ACP results show 47% and 51% one-shot incapacitation respectively.

According to this particular study, even the "larger holes are better than smaller holes" statement would go out the window.

Something seems wrong when .32 ACP/.32 Long looks like the clear winner in a comparison of handgun cartridges. If that means I'm "bashing" Ellifrizt's work, then I guess I'm bashing it.

What it means is that you didn't understand Ellifitz's analysis of the data.

First, look at the article and the chart there labeled "Failure to incapacitate."

Then have another look at post 9 where I quote Ellifritz:

Originally Posted by Greg Ellifritz

...Take a look at two numbers: the percentage of people who did not stop (no matter how many rounds were fired into them) and the one-shot-stop percentage. The lower caliber rounds (.22, .25, .32) had a failure rate that was roughly double that of the higher caliber rounds. The one-shot-stop percentage (where I considered all hits, anywhere on the body) trended generally higher as the round gets more powerful. This tells us a couple of things...

In a certain (fairly high) percentage of shootings, people stop their aggressive actions after being hit with one round regardless of caliber or shot placement. These people are likely NOT physically incapacitated by the bullet. They just don't want to be shot anymore and give up! Call it a psychological stop if you will. Any bullet or caliber combination will likely yield similar results in those cases. And fortunately for us, there are a lot of these "psychological stops" occurring. The problem we have is when we don't get a psychological stop. If our attacker fights through the pain and continues to victimize us, we might want a round that causes the most damage possible. In essence, we are relying on a "physical stop" rather than a "psychological" one. In order to physically force someone to stop their violent actions we need to either hit him in the Central Nervous System (brain or upper spine) or cause enough bleeding that he becomes unconscious. The more powerful rounds look to be better at doing this....

Then also look at post 9 where I quote the FBI paper entitled "Handgun Wounding Factors and Effectiveness", by Urey W. Patrick, page 8:

...Psychological factors are probably the most important relative to achieving rapid incapacitation from a gunshot wound to the torso. Awareness of the injury..., fear of injury, fear of death, blood or pain; intimidation by the weapon or the act of being shot; or the simple desire to quit can all lead to rapid incapacitation even from minor wounds. However, psychological factors are also the primary cause of incapacitation failures.

The individual may be unaware of the wound and thus have no stimuli to force a reaction. Strong will, survival instinct, or sheer emotion such as rage or hate can keep a grievously wounded individual fighting....

In other words, much, or even most, of the time a person when shot will choose to stop what he's doing for psychological reasons. He is not actually incapacitated, but rather he decides to stop because it hurts, or he's afraid, or he's convinced himself that once he's been shot he's incapable of continuing, etc. In such cases, what he's been shot with really doesn't matter. It's enough that he's been shot and he knows it. His mind then tells him not to continue, or his mind tells him he can't continue. His mind decides not to continue, even though he may in fact be physiologically capable of continuing.

But what if the assailant's mind doesn't know or care that he's been shot? That certainly seems to happen at times. And in such cases, the assailant isn't going to stop unless/until his body is sufficiently damaged in such a way that he is forced to stop. So he might be forced to stop because his brain or spine is physically damaged. He might he might be forced to stop because major bones/skeletal structures are smashed to the point that he has lost mobility. He might be forced to stop because he has lost so much blood that the oxygen level in his brain has fallen to the extent that it can no longer function.

The thing is that if you need to shoot someone in self defense you can't know ahead of time whether he will stop because of psychological factors or you will need to damage him enough to make him physiologically incapable to be a continued threat. Bullets making more and larger holes deep enough in the right places are more likely to produce a sufficient physiological response if the assailant is not willing to just stop because he's been shot.