How’s that for an attention-grabbing title?  Yeah, I know, sounds like no need for sleeping pills, it sounds like this article will put you right out, right?

Well, hopefully not so.  Hopefully this can shed some light on a new factor I’m introducing to my YouTube channel’s ammo tests: The MacPherson Wound Trauma Incapacitation (WTI) Factor.

I discussed the WTI factor in a previous blog article, http://shootingthebull.net/blog/how-effective-is-a-hit-from-a-judge/.  But I wanted to address it more head-on and more specifically, here in this article, so that when people see the end of my ammo tests and see a “MacPherson WTI Factor” quoted, they’ll know what it means.

What Is The WTI Factor?

In the simplest possible terms, the MacPherson WTI factor is an attempt by Duncan MacPherson, author of one of the seminal works on wound ballistics (“Bullet Penetration“), to quantify just how effective a bullet would be in stopping an attacker.  Now, there have been many attempts at doing this before; an example would be the TKO (“Taylor Knock Out Factor”).  The problem with prior models of bullet effectiveness is that they didn’t actually take into account what makes a bullet effective!  Instead they relied on (seemingly arbitrarily) assigning factors to caliber, diameter, speed, weight… all the right things, yes, but without knowing and modeling how they work together, and without knowing what actions of a bullet actually STOP an attacker, how can you rank them?

Voluntary Incapacitation

Let’s go through the reasons why an attacker would stop attacking.  There are many, but they basically break down to two categories: voluntary, and involuntary.  Voluntary factors include:

1. They see that you have a gun, and they decide to call it quits right then and there.  If not, then:
2. They see a gun pointed at them.  That can be a real attention-getter, staring down the barrel of a loaded gun.  It might be enough for them to decide that they’d be better off somewhere else, and fast.  If not, then maybe when:
3. They hear a gunshot, or see the muzzle blast.  Now, they may or may not have gotten hit, but just hearing the shot or seeing flames erupt from the front of a gun can be enough, in some cases, to get an attacker to drop what they’re doing right away.  If not, then perhaps:
4. They get shot.  This one starts to cross the line between voluntary and involuntary, but let’s stay strictly voluntary here — let’s say they get shot, with a flesh wound.  Nothing serious.  Maybe they got grazed.  Maybe they got shot in the leg.  Doesn’t really matter, all I’m after here is: it’s not life-threatening, but maybe it’s bleeding.  That right there will stop many attackers; many people will choose to stop their attack when they feel the panic of “I’ve been shot — I need to get to the hospital immediately or I’m gonna die!”

Okay, that basically covers the voluntary reasons someone might choose to stop an attack, in progressive order.  But what if they won’t?  What if they simply will not stop attacking, even after being shot?  Maybe they’re high, or so enraged, that they just don’t care or don’t even notice that they’ve been shot at all.  What then?  Well, at that point you will be relying on your defensive gun to FORCE them to stop attacking.  And that’s where you need to know how effective your bullets will be in causing wound trauma, and what level of wound trauma you’d need to be able to inflict in order to force even a determined attacker to stop.

Sometimes, Any Bullet Will Do

A side diversion here — sometimes, really, any bullet from any gun will do equally as well as any other bullet from pretty much any other gun.  Look through the list of “voluntary” reasons: in those scenarios, it very likely wouldn’t matter what bullet or what caliber you had on hand; if someone’s going to be stopped by hearing a gun fire, it’s not very probable that they’ll be thinking “oh, wait, that was only a .380, never mind, I’ll keep attacking.”  Instead, it’s pretty much the case that if someone sees a gun pointed at them, they’re not likely going to process the barrel size as part of their decision-making process.  Even the process of getting shot — obviously a big bullet would cause more wound trauma than a small bullet, but if we’re discussing the psychological reasons of when attackers stop attacking, then when it comes to a flesh wound, it probably wouldn’t make that much difference to an attacker.  Seeing blood pouring out of their bodies will be what forces their decision; they’re not likely to go measuring the hole in them to see what size of bullet they got hit by.

So this is good news, for those who advocate smaller pistols and smaller bullets — yes, there are many cases where they will be equally effective in deterring an attacker.  But (and, as Pee Wee Herman said, “everybody’s got a big but”… what about when you need to force them to stop?  What if you need to invoke an involuntary incapacitation?

Involuntary Incapacitation Factors

Sometimes the bad guy isn’t going to cooperate.  Sometimes they won’t go away.  Sometimes, you may need to use deadly force to bring the attack to a halt RIGHT NOW.  Sometimes you need to take away an attacker’s capacity to attack you — you need to take away their ability to attack you.  This is what we’ve been referring to as “incapacitation” — when the attacker’s ability to attack is taken away from them.  This can be through many different ways, including rendering them unconscious, or even dead or paralyzed.  Regardless of the method of forcing them to stop, this section is about the hard business of making them stop immediately.

There are two basic ways to bring about involuntary incapacitation — either through an attack on the vital structures of the body, or through overall collective damage to the body that the body just shuts down.

Attacking the vital structures is called making an “incapacitating hit”.  Damaging the vital structures (such as the brain stem, spinal column, or destroying something in the circulatory system that causes a big drop in blood pressure) will cause an attacker to stop immediately (in the case of a brain stem or spinal column injury) or will cause them to stop very soon (in the case of hitting an artery or the heart).  Those types of hits will interrupt the flow of blood and therefore will soon deprive the brain of oxygen, although this is not immediate; even in a case of completely destroying the heart, the brain may have enough oxygen to continue attacking for up to about 10 seconds.

Scoring an incapacitating hit on a determined attacker is no easy task.  Bullets are small, and the vital areas on a human body are also quite small; the spinal column is maybe 2″ in diameter.  In order to hit these structures you’ll need immaculate shot placement (more on that later)  Here, again, the caliber doesn’t really matter much — a .22LR hit to the brain stem will stop an attacker immediately, just like a .45 ACP or a 10mm hit to the brain stem.  It’s true that the bigger bullet will have a higher likelihood of hitting something than a smaller bullet would (meaning, the larger the bullet, the more chance it would have to turn a “near miss” into a “partial hit”) but, assuming your shot placement is perfect, then any bullet (that penetrates deeply enough) can get the job done.

The Myth Of Shot Placement

So now we get to the controversial part.  Scoring an incapacitating hit on a determined attacker requires immaculate shot placement.  And just about the only way to get that is dumb luck.

Luck?!?!  Yes, I said it.  Luck.  Because if you manage to place that bullet exactly where you wanted to, in the heat of an absolute life-or-death situation where you are facing imminent death (or great bodily harm, as the law allows) and your attacker is moving rapidly — well, good luck with your shot placement.

It’s often said “Shot Placement Is King.”  Yes, it is — that is absolutely true.  The problem is, the odds of you being able to control your shot placement to where you want it, are very low.  Think about what’s going on — you’re not facing a paper target.  You have a living, breathing human being, who isn’t standing placidly, they’re trying to kill you.  You’re going to have the mother of all adrenaline dumps going on.  You’re going to be gripped in the most severe case of “Flight Or Fight” that you will ever experience.  Your fine motor skills are going to go by the wayside.  You’re going to have tunnel vision.  And you’re going to have barely a second or two to draw, aim, and fire.  Oh, and your attacker is bearing down on you, moving as fast as he can — and moving targets are always much harder to hit.

Still feeling absolutely confident about your shot placement?  Still think that you’re going to hit that 2″ wide spinal column?  I wouldn’t be so sure about that.  I wouldn’t want to rely on my ability to bring about a “one shot stop” in such a scenario!  And I certainly wouldn’t want to count on my ammo because some guy wrote in some column that “this is a great round, it dumps a lot of kinetic energy into the target” or whatever.

Accordingly, MacPherson wrote a 300+ page book to describe exactly what bullets do, how they behave, what they do to living tissue, what effects will stop someone immediately, and so on.  The net result is a formula that MacPherson predicts will have the potential for stopping an attacker, through general tissue damage — i.e., not relying on an incapacitating hit to the central nervous system.  He bases his formula on the general idea of overall tissue damage causing the body to shut down, as verified in an old Thompson-LaGarde test where steers were shot in non-vital areas with various calibers until they dropped.  It can be presumed that the steers weren’t influenced by any of the psychological factors, because the steers wouldn’t know what a gun was, wouldn’t know that having been shot meant they needed to get to the hospital, etc.  And, by avoiding the central nervous system or circulatory system, they showed that general damage to the body in sufficient quantity would be enough to cause the steer to drop.

So How Do I Use This?

In MacPherson’s work, he produced a formula that takes into account the penetration depth, the amount of tissue destroyed, and the type of bullet (a hollowpoint, a round-nose FMJ, a buckshot ball, etc) and comes up with a mathematical number that represents the amount of vital tissue that will be destroyed by that particular bullet.  MacPherson says that about 40 grams of tissue would need to be destroyed in order for the body to undergo such shock that it might shut down (again, there’s no guarantee, but this is about as good as he could predict).  40 grams of tissue is about as much as a hot dog, and I think you could imagine how substantial an injury would be if someone carved a complete hot dog out of your body!

In my ammo tests I’ll be reporting the MacPherson WTI (Wound Trauma Incapacitation) Factor, which will let you get a general idea of how one bullet compares to another when considering how much tissue it damages.  The MacPherson WTI is biased away from shallow-penetrating flesh wounds (for example, a really big-diameter bullet that only penetrates a few inches, would rate a zero on his scale; his scale requires bullets to reach deep enough that they’re not just damaging muscle, but are reaching the deepest internal sections of the body).  It also penalizes overpenetration; if a bullet just zips right through the attacker and wastes its energy by exiting the attacker, then he only credits the bullet with the damage it would have done within the attacker.  Obviously, an overpenetrating bullet won’t be as effective as one that puts all its available energy to work damaging as much tissue as possible.

It is in this arena, the involuntary incapacitation through tissue damage arena, where caliber becomes much more important.  For voluntary cessation of activities, any caliber will do.  And for critical hits on the central nervous system or major blood vessels, a hit by any caliber will be effective.  But when it comes to stopping an attacker without those critical hits, then the bigger, deeper-penetrating bullet that damages the most tissue is the one that will bring about involuntary incapacitation sooner.

MacPherson’s level of damage necessary for incapacitation is about 40 grams.  In the .380 ACP hollowpoints I’ve been testing, most are delivering MacPherson WTI factors of around 18 to 23.  What this means should be pretty obvious — don’t go betting your life on a “one shot stop” with a .380!  It’s going to take at least two or three good solid hits before you’ve damaged enough tissue in the attacker that they may be forced into an involuntary shutdown.  CAN you stop an attacker with one shot of a .380? Of course, if you have perfect shot placement and you hit a vital structure or the central nervous system.  But considering that the odds are quite against that, it would be wise to not rely on a “one shot stop”.  Instead you should count on the idea that you’re going to have to “shoot until the threat stops” — and that will likely mean at least two or three shots.  Now, I’m not going to guarantee to you that someone will stop after getting hit in the torso with two or three .380 ACP hollowpoints!  Obviously every shooting scenario is different, and the performance of bullets can vary from shot to shot, and it would also depend on whether the bullets hit something important (a vital organ) or just passed through muscle.  But in general, if you do your part and place the shots at least in the main torso, destroying 40 grams of tissue is perhaps going to bring about incapacitation even if you don’t get that rare CNS or artery shot.

Bigger bullets and bigger calibers make this easier, obviously.  Many .45 ACP hollowpoints will deliver a MacPherson WTI of about 55, meaning that it’s possible (not saying probable, but possible) that one solid shot to the body might bring about incapacitation.  Even then, I would still recommend following the advice of your firearms instructor when she says “shoot until the threat stops”.  But the larger bullet will give you a higher likelihood of hitting one of the critical structures in the body, and it will also destroy more tissue which may lead to faster incapacitation even in cases where it doesn’t hit one of those critical structures.

Reporting the MacPherson WTI will let you gain a better idea of how effective the bullets I test may prove to be if you ever need to use them.

In Summary

When selecting your defensive ammunition, here are the factors as I see them:

1. Any bullet, placed perfectly and penetrating deeply and hitting the central nervous system (CNS) or a major artery or the heart, will stop an attacker just as well as any other bullet would.  In this case, caliber does not matter.
2. Under the stresses of actual combat and the massive adrenaline dump that goes on, and the nature of a moving target who is attacking you and everything else that’s happening… there is practically no way that the average shooter is going to place that bullet perfectly.  It just will not (likely) happen.
3. Knowing the results of the Thompson/LaGarde tests, and knowing that there were no psychological factors involved that caused the steers to drop, it seems obvious that with enough tissue destruction, even hits in nonvital areas CAN cause incapacitation.  They won’t always do so, but they could.
4. Given the uncomfortable but clearly obvious truth of #2 above, it seems prudent to have a backup plan — being, have on hand the capability to administer enough non-critical wound trauma that you can still incapacitate the attacker even without that miracle CNS shot.

Place your shots as well as you can.  Practice as much as you can.  Practice speed and accuracy, practice moving while shooting, practice practice practice.  But when it comes to betting your life on being able to hit a tiny spinal column in a moving attacker while you’re moving and dodging and your fine motor skills are gone due to a huge influx of adrenaline… well, let’s just say that I wouldn’t want to rely solely on my ability to make that miracle shot.  Knowing how bullets work, knowing how they damage tissue, and knowing how much tissue damage is necessary before involuntary incapacitation might come into play, prepares you to be better equipped to successfully defend yourself if the time ever comes.