My 2 cents on hand gun calibers and what works for stopping bad guys

In the gun world, there's been an endless debate for MANY years about what caliber and bullet works best for stopping an attacker. When I started to carry a handgun, I did a lot of reading and research into what works and what doesn't regarding this.  I do not claim to be an expert, but I feel my knowledge may have some value to others, hence the purpose of this write up.  Keep in mind that this is written towards handgun calibers, and NOT rifle calibers.  

Americans historically tend to larger calibers in hand guns.  The US army switched to 45ACP in the early 20th century based on their experiences in the Philippines. Police forces in the US have migrated from the 38 spl to the 357 mag, then later on  9mm, 40 S&W, and 45 ACP. So which one works the best and why?

Currently, there seems to be 2 big theories in "stopping power".  I am defining "stopping power" as a bullet's ability to inflict damage on an attacker and to stop him from being able to continue his attack.  Stopping power can vary hugely based on hit location, caliber, type of bullet (FMJ vs JHP), velocity of bullet, and probably a few other factors I am overlooking.  For the purpose of this document,  I am assuming all external factors (external to the round being used) are equal (hit location, size and mental status of bad guy, distance from good guy to bad guy, etc) between the rounds being compared.

The first theory, championed by such individuals as Marshall and Sanow, among others, is the "energy" theory.  The basic premise is that the bullet stops a bad guy by dumping energy into the target, this energy then disrupts vital tissue, causes pain and shock, and generally disables the hostile.  Energy theorists tend to value light weight high speed bullets for a given caliber.

The second theory, first championed by Dr Martin Fackler (a Vietnam trauma surgeon and later head of the Army's Ballistics labatory) and the FBI, is that a handgun bullet's stopping power is determined by only 2 factors.  Depth of penetration (how far it buries into your target), and how much tissue it crushes during penetration
(aka, how big of a hole).  

I tend to dismiss the energy theory due to the poor data quality provided by theorists of it.   The studies I've read and seen backing it usually have poor methodology (Marshall and Sanow in particular have been savaged due to the gaping flaws in their studies) and don't seem to match up with what happens in the real world. Probably the biggest argument against the energy theories is the excellent and well document research by Dr Fackler, and the FBI's research following the tragic Miami shootout of 1986.  As a side note, many of the gimmick bullets on the market (Extreme Shock ammo being an infamous example of this) use the energy theory.

So that leaves us with the Fackler method.  Meaning, drill a hole into your target that’s as big and deep as possible.  What makes the hole big?  The bullet expanding as it buries inside.  What makes it go deep?  Bullet weight and velocity.  All things being equal, a small diameter heavy bullet traveling at high speed will go deeper then a large diameter light weight bullet traveling at low speed.  The current "standard" for penetration as set by the FBI is 12 inches.  Less penetration then this can result in a bullet not burying deep enough into a target and damaging vital structures (as happened during the 1986 Miami shooting).  More penetration then this isn't necessarily harmful, but too much can mean that you’re losing expansion potential (see below) and having your round exit the bad guy.

Expansion with JHP's can get tricky.  Expansion is usually determined by how the JHP is designed, with velocity playing a part as well.   Historically it’s been desirable to have more velocity (which helps with expansion usually), but the latest generation of JHP's can expand nicely even at lower velocities.   Sometimes too much velocity will cause a JHP to fragment, or expand too much or too quickly, causing it to not penetrate deep enough.  Too low of a velocity can result in under penetration and under expansion (worst of both worlds).  Given equal factors and bullet design, the bigger the base caliber, the bigger the expanded bullet will be.  Most JHP's will expand between 1.5 and 2.0 the size of the base caliber.  This is why it’s usually EXTREMELY foolish to use FMJ's for carry ammo.  No expansion, and FMJ's (except in the small "poodle shooter" level of calibers) tend to exit right through your target and continue downrange.  This is why virtually no police department in the US carry FMJ's in their handguns.

So in a perfect world, a given round would penetrate to 12 inches or a bit deeper, expanding as large as possible.  All of the handgun calibers from 9mm and up can penetrate this deep (or more) depending on the bullet used.  So the challenge is to get maximum expansion while maintaining the desired penetration.  If that’s all that mattered, the perfect round would be a 45 acp JHP that makes a foot deep hole while expanding to about .9 inches.  

However, there's a twist (as always).  45 acp guns tend to be larger, heavier, and harder recoiling compared to guns in 9mm and 40 S&W, while holding less ammunition (though some credible arguments exist that the recoil pattern of 40 S&W is snappier then 45 ACP).  What’s better?  To make a few bigger holes in a given time frame, or make more smaller sized holes?  Carry a gun that holds 12-17 smaller caliber rounds vs one that holds 7-13 larger ones? There's no "right" answer here. Every shooter will have to decide this for themselves.  

Ok, so now you've chosen your gun and caliber.  What should you feed it with?  This is probably the biggest religious debates on the various gun forums.  My advice is to look at well documented bullet testing conducted by such places as FBI testing, police workshops, etc.  Be extremely wary of manufacter's claims unless they are WELL verified by independent outside sources, especially if the claims are really outrageous (i.e. Extreme Shok claiming their rounds cause neurological collapse). Conducting your own testing isn't a bad idea either.  Google for "wetpack testing", get some ideas, and then try it for yourself.  

Finally, your probably wondering what type of gun and ammo I carry.  For the curious, its a Glock 26 loaded with Federal HST 147gr.  My testing with it can be found at the following link.