Stop me if you’ve heard these terms before.
These are all terms that have no relevance whatsoever in terms of armed self-defense. They’re myths– terms kicked around by people who are used to hunting large, heavy game and see animals ‘go down’ from a distance without an appreciation for the forces involved.
Knockdown is definitely one of the worst terms to use around gunfights. If you’ve ever seen someone get hit with a bullet, of any caliber, you know that they don’t take a Hollywood pratfall- it whips through them like a rock through cheesecloth, and at /most/ they might stagger and fall down. And yet, it is such a staple of the American lexicon, that even people who’ve never been around guns in real life assume that a shotgun blast will knock a standing man through a window (dramatically, at that). If you want to see knockback at work, take a look at a beanbag round hitting a human body– it knocks them down because the deceleration of the bag is comparatively much, much shorter than a round going through-and-through.
Remember your basic physics, F=m*a; if you shoot someone with a projectile that goes through them, it’s only going to accelerate them with the same Force that the bullet lost. Unless the target’s wearing full body armor and gets hit square in the chest with a whopping heavy round (I’ve seen video of this), there’s just not enough acceleration from a physics standpoint to deliver the sort of hit that will actually ‘knock’ someone down. Think about taking a punch on the chin– leaning back and ‘rolling’ with the punch vastly reduces the impact, whereas leaning into the punch even a very little bit can turn it into a ‘knockout’ blow. A similar principle applies to terminal ballistics, where lead meets very elastic flesh and muscle and fluid.
‘Stopping Power’ is the idea of a one-shot-stop. You hit the target once, they fall down and are unable to continue the fight. Stopping power is a dangerous myth because outside of very heavy, high-velocity and high-mass loads, accuracy is a much more important metric than raw kinetic energy. A certain critical threshold of penetration is required to guarantee a hit to the Central Nervous System (I touched on that in a previous article) and a certain amount of bullet expansion is critical to guarantee exsanguination. These are the only two surefire ways to stop a determined aggressor, by physically disabling their ability to continue the fight. Simply lobbing a big bullet at someone is a hardware solution to a software problem, and is no substitute for clean high-center mass hits.
So when people talk about ‘stopping power’ they’re referring almost certainly to a bullet with enough speed and weight to smash through the human body with enough force to knock the brainstem, spine, or heart into disarray. The human body is phenomenally resilient, and barring a lucky hit to a major tendon or nervous cluster, it will continue to move and respond even after being hit with multiple bullet impacts. Even a straight heart shot that completely perforates the aorta can leave an attacker up and moving for 3-7 seconds– in some cases, people have not only survived heart shots, but lived to fight off their attackers and then survive surgery afterwards with few ill effects. Many people point to statistics of ‘2-3 shots of 9mm versus 1 shot of .45’, but they’re missing that key metric of 3-7 seconds; firing a Glock rapidly, it’s easy to put 3 rounds center-mass on someone while a big, slow-moving .45 might tag them just once.
There is much made of ‘crushing power’, a round’s ability to smash through bone and dense tissue, but the chaotic nature of fluid dynamics and relative flexibility of bone makes it impossible to say authoritatively that any given handgun round is superior to any other. Some .45s and .357s bounce off skulls; some 9mm rounds perforate arms, ribs, and clavicle and keep on trucking. In any case, the amount of force necessary for a badly aimed shot to peripherally incapacitate the CNS is beyond excessive– a .44 magnum approaches that level, but is bad in all measures as a combat round due to excessive recoil and low ammunition capacity. Considering that even veteran cops have seen gunfights with less than 10% accuracy out of 100+ rounds fired, 5 rounds of .44 might be staunchly inadequate in a fight.
The natural segue from Stopping Power is Hydrostatic shock; the idea that the soft tissue in a target stretches and takes enough damage that the propagating shockwave of impact will destroy the attacker’s internal organs. This is one of the most widely debated points in terminal ballistics, and there are proponents and detractors on both sides. I will make a few observations here (FTR, I do not agree with the theory): the first and most important part of this theory observes that HSS seems to primarily occur when significant displacement and temporary cavitation occurs in a target. I’m talking about a bullet that’s carrying a tremendous amount of physical energy, like a full-sized hunting round being pushed up to incredible velocities, and hitting a target hard enough to deliver 100% of that kinetic energy into the target. A cape buffalo is several feet from shoulder to shoulder; a human, barely a foot from sternum to spine. It’s entirely possible that hydrostatic shock is a ‘thing’, but it seems very likely that it’s simply not a relevant factor in the human body because the proximity of major blood-bearing organs to CNS structures means that any shot placed close enough to the heart/lungs/spine to incapacitate an attacker with HSS is likely going to cause exsanguination, and probably at a similar rate. I submit that it’s just as likely that the rapid transfer of energy and severe internal trauma caused bruising akin to whiplash in the target.
You can go digging into principles of all the above theories, and you’ll find their detractors and supporters on the interwebs. I think it’s worth pointing out this: if you’re hellbent for leather on finding someone, anyone, who agrees with your presuppositions about ballistics, then this probably isn’t the blog for you anyway. The FBI doesn’t consider hydrostatic shock theory with enough weight to support their weapon selection process, and the ideas of stopping power and knockdown haven’t stopped the FBI from returning to 9mm after decades of flirting with .357, 10mm, and .40 S&W. It meets the only criteria that matter: sufficient penetration, expansion, and cavitation. Anything else that occurs in a gunfight, whether magic pixies or hydraulic forces, are simply not as reliable as accurate, well-placed shots high center mass.