Gun Science: What’s “Knockdown” Power?


OK, so that was a trick question because there’s really not much to “knockdown power” at least in the literal sense. Sure, a cannon would certainly knock you over, but a handgun? Not so much. Let’s explain.

On one of those "let's try this" days, we put a bulletproof vest on a 50-pound bag of sand and shot a bunch of handgun rounds at it. No caliber could even come close to tipping the bag over.

On one of those “let’s try this” days, we put a bulletproof vest on a 50-pound bag of sand and shot a bunch of handgun rounds at it. No caliber could even come close to tipping the bag over.

Handguns are certainly lethal and can cause all sorts of reactions in the recipient of a handgun shot. One who catches a handgun round might fall, jump, flinch, or do absolutely nothing at all. And therein lies the myth. There are lots of factors that cause physical reactions to a handgun shot. Among them are fear, pain, structural damage, or “electrical” damage just to name a few. What doesn’t happen is a physical “knock you over” force.

To grossly oversimplify the physics involved, think of two factors: kinetic energy and momentum. Kinetic energy, usually expressed as foot-pounds for ammunition, can be thought of as potentially destructive energy. Think about a a high-speed drill bit. That will certainly cause a ruckus in a piece of soft wood, but it won’t send that lumber flying through a plate glass window, or “knock it down” if you will. The other “energy” component is momentum. Think of that as the ability to physically move an object. When you whack one of those wooden balls with a Croquet mallet (OK, so few of us probably do that, but you get the point) the ball goes flying across the meticulously manicured lawn.

Back to handgun bullets. Even the venerable .45 ACP bullet has very little momentum. While there is plenty of kinetic energy (drill bit analogy here) there is not enough physical momentum to move anything of significance. If you take away the pain, fear, structural, and electrical damage factors, it’s just not going to “knock over” a 200-pound person. Yet people say it has knockdown power.

A 230-grain .45 ACP bullet moving at 850 feet per second generates 27.9 pounds-feet per second of momentum. To put that in perspective, a fastball tossed by Nolan Ryan delivers almost double that amount at 52 pounds-feet per second. Sticking with a sports analogy, if you get whacked with a golf ball hit by one of the PGA’s power-drivers, Bubba Watson, you’d experience a bit more momentum impact with 28.9 pounds-feet per second.

While a .45 ACP round did a number on this can of spam due to kinetic energy, it doesn't have enough momentum to physically knock someone down.

While a 9mm round did a number on this can of spam due to kinetic energy, it doesn’t have enough momentum to physically knock someone down.

It’s just the math. Don’t believe me? Next time you have the opportunity to shoot at an outdoor range, try shooting a couple of inanimate objects of different weight (be safe!) and see what happens. You might be surprised. That’s why it’s so important to be prepared with the skill to make shots count and to have the equipment in terms of spare magazines to account for the unpredictability of handgun effects on targets.

Tom McHale is a committed learning junkie always seeking a new subject victim. As a lifelong student of whatever grabs his attention on any particular day, he thrives on beating rabbit trails into submission. In between his time as a high-tech marketing executive, restaurant owner, and hamster cosmetology practitioner, he’s published seven books and nearly 1,500 articles about guns, shooting, and the American way.

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  • Doak Carswell

    There was a recent video comparing 9mm to 45acp in terms of ” knockdown power “. The shooter was a using silhouette knockdown target. He used , I think, 124gr 9mm and 45acp 230gr. The shooting started at the bottom of the target, and he worked his way up, until the target finally fell. The 45acp 230gr was always able to knock the target down first over the 9mm. I personally carry 9mm for PD, usually, and sometimes 45acp. But I will have too say that this video showed me that there is a difference in actual knockdown power. JMHO

    • Tom McHale

      Of course. Different calibers have different levels of momentum. But if that knockdown target was a 175-pound object, none of them would have “knocked it down.”

      • Doak Carswell

        Well, I weigh about 210, and I really wouldn’t want to stand in front of either to find out. Especially the 45acp. Don’t know about you.

  • dprato

    When you shoot someone all of the other factors are in play so it makes the content of this article moot.

    • Tom McHale

      True, but the internet if full of stories and claims of certain calibers being capable of knocking someone through a plate glass window, over backwards, or right off their feet, hence the article. Hollywood doesn’t help kill that “knockdown” myth…

      • dprato

        The internet also has nothing to do with reality.
        If I shoot you in the head with a 9mm you will drop & if I hit you in the chest with a 357 mag you are going down & the mere fact you discount all other factors makes the article meaningless.

        • CZFan

          Getting shot in the head with a pellet gun can be lethal.

          Shooting someone in the torso with a .357 or .44 magnum is not a “sure thing” anyone who says otherwise is a fool.

          Shooting someone in the Heart or Spine is a sure thing. Caliber has nothing to do with it.
          And if you have ever shot a .357 mag without ear protection in a small room you will know that everyone inside will be stumbling around trying to restore equilibrium, while making idiotic “Mah, Mah, Mah,” sounds trying to get their ears to pop.

          Most people call a 9mm “puny” and too small for Bear Country. I go to Alaska every year for Salmon Fishing, Both my Dad and I bring 9mm’s.

          I am partial to my CZ85 he likes a SIG p226.

          Why on earth would I deliberately carry a 9mm when attack from a 800+ bear is a real possibility? Experience.

          In 1977 my uncle was Bow hunting Elk in Alaska, he slipped down a hill right into the back of a bear foraging.
          The bear was not pleased, it roared, he let an arrow loose, that made it even more angry and it charged.

          The first hit ripped his left forearm open breaking both bones and a few ribs while knocking him onto his right side covering his full flap holster.

          The bear started batting him around on the ground and bouncing on him, breaking more ribs, and alot of bones in his right arm and hand.

          He got to his Browning Hi Power and squeezed the first shot off as soon as he could, He said he knew he was hurt before he shot but he didnt realize that his right arm was demolished, broken radius and ulna, broken and dislocated fingers and other small damaged bones. On top of similar damage to his left and a smattering of broken ribs.

          His first shot missed but the bear stopped beating him to death and stood up on its hind legs presumably to finish the job.

          And that allowed him to put the other 12 into the thing.

          Most shots went in through the bottom ribs or under them and punched through the back ribs after turning its organs into soup.

          Since this was the 1970’s there was FMJ military ball for the 9mm and thats about it. Thats what he used.

          He said that shooting in that situation was like having his body lit on fire and in no uncertain terms that had he been carrying a gun like a .357 Mag or something “better for bears” he couldnt have fired more than once or twice.

          While fishing I have seen some pretty stupid guns that people carry for “bear protection”

          Ive seen guys with S&W 500’s Taurus Raging Judges and other insane guns with in Calibers like .460 Rowland, .454 Casull and even one asshole Salmon Fishing with a .475 Linebaugh.

          One guy carried a shotgun slung over his back. I respected that choice If you are going to carry 5+ lbs of gun you may as well go big or go home.

          They all snicker and laugh at my 9mm saying “that cant stop a bear its not powerful enough” Yeah maybe 1 9mm wont stop them but all 17 of his brothers will.

          My favorite is have them shoot a paper plate at 7 paces. They can shoot however they want, one hand two hands, whatever. Its ALWAYS slow.

          When they see someone put 18rds on target faster then they shoot 6 all snickering stops.

    • CZFan

      Plenty of people think that they “know guns” Ive heard alot of them say a .45 ACP will send someone flying when hit in the chest, and the “puny 9mm” wont do anything.

      They also think Shotguns shoot people across the room with force akin to an air cannon.

      I even think the Myth busters hung pork rib inside of various soft and hard armor vests and had them just barely on a hook holding them up. Anyone could give the vest a tap and it would fall.

      Everything from a .22 to a 12 guage slug failed to move the vest and pork an inch.

  • Dr. Ron Polland

    ‘Knockdown power” is a misnomer whereas “stopping power” is not. Stopping power is simply the power needed to stop a threat. Considering that merely brandishing a firearm has ended a lot of threats makes the caliber of that gun totally irrelevant.

    If you actually have to shoot a person who poses a credible threat to your life or to others, then it has to hit him where it stops whatever he is doing at the moment you pull the trigger – starting with breathing and then thinking. i’m a big fan of a quick double tap to the heart followed by a head shot. You don’t need a massive bullet to penetrate a skull as plenty of Mafia hit men and Mossad agents will testify to the power of the mighty .22 to turn gray matter into guacamole.

    This is the golden age of exotic and toxic ammunition where a bullet like the G2 R.I.P. (R.I/P. stands for “Radically Invasive Projectile”), even in .380ACP, is made up of individual pointed trocars (like spear tips) that make the bullet easier to pierce through flesh. After hitting a human target (as simulated with ballistics gel or a ham hock), the bullet dumps all of its energy within the first 6″ to 8″ while sending those trocars flying off in all directions from the point of impact ripping apart flesh as they go. Gun owners say that the expanding bullets prevent over-penetration (shooting through a target, or, say, the walls of your house) and make sure the bullets don’t ricochet. But these R.I.P. rounds also cause horrific wounds in soft tissue.

    Here’s a video of what this sucker does to a block of ballistics gel (and a watermelon):

    These rounds cost $2.00 a pop but all it would take is one well-placed pop to stop a perp in his tracks. Then again, isn’t your life worth a couple of bucks? Because those trocars are razor sharp, I do feel sorry for the surgeons who have to work on the perp to keep him alive and/or the medical examiners who have to do the post-mortem on the corpse. But, not that sorry.

    Then we have available rounds (like the Ruger ARX or Lehigh Extreme Defense) that are solid bodies of metal but are crafted into unique patterns that cause the bullets to spin at very high speeds as the bullets pass through the air on their way to a target. When they hit soft tissue, they enter like FMJ’s but that spin makes the sculpted bullets like the blades of a blender or a Roto-Rooter carving massive wound cavities as they go.

    Lehigh Defense:

    Ruger (Polycase) ARX:

    In short, these new rounds provide a lot of stopping power: they will stop a perp from proceeding to do bodily harm to you and your loved ones.

  • j turner

    Did you do video of shooting the spam?

  • Luca Brasi

    Let’s face it, a hit with a .22 is deadlier than a miss with a .44 magnum.

  • john shirley

    Good explanation.

  • Greg Foster

    Good article, like the comparisons.

  • Brian Mooney

    Good, accurate, and entertaining article. I especially enjoyed it as I have had to try to teach many of the basic ideas of physics in 35+ years as a college science professor. If you want another challenge, try convincing a roomful of folks that a vacuum doesn’t “pull” anything” -it is one of the main reasons my (remaining) hair is grey.

    I think some commentators still managed to miss the point you made so vividly. Oh well. At least that Spam still looked pretty good! All that pink reminded me of the late “Gunny” on TV hitting a row of watermeons with a Gatling gun. The melons were pulped, but all stayed in placed just as he expected. He understood ballistics. Gunny observed that he felt that the Gatling “tenderized” the melon’s flesh. I’m sure going to miss that guy!

    • Tom McHale

      Well, as I’m sure you will agree, there is an exception to the laws of physics. The .45 ACP is so powerful that it “pulls” you off your feet and into the air 🙂

  • Capt.Steve Thompson

    Spam, Spam AGAIN!

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