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Knock Down Power: Myth vs Fact

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I’m sure all of us have had a debate regarding knock down power of any given cartridge. Whether it be online, in person or otherwise, most of us have probably bought into the fact that cartridge A has more knock down power than cartridge B and so forth. I know I’ve debated it myself and I’ve read facts from cartridge manufacturers until I couldn’t keep my eyes open. What can we learn from published facts regarding knock down power and how it translates into making informed decisions on what we carry?

First off, I think it’s important we try to establish a baseline. What is knock down power? In short, it’s the energy produced or in this case delivered to a target. Want to get scientific? Newton’s 2nd Law of Motion deals with energy. It states The vector sum of the forces F on an object is equal to the mass of that object multiplied by the acceleration vector a of the object: F = ma. In short, the speed and mass at which an object travels determines the energy is imparts. A heavier object traveling at the same speed as lighter object will impart more energy. This law is exactly why it’s so important to get accurate information on the cartridges you’re going to carry that have been tested in the barrel length of the firearm you carry. An published velocity of 1200 feet per second out of a 5 inch barrel means very little if you carry a 3 inch barrel. Energy will be less. Thank you to Mr. Newton for his help with this topic!

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Let’s dig a little deeper on Knock Down Power. When I chose to purchase my Glock 29 Gen 4 in 10mm Auto, I based my decision upon a few factors. I wanted something that could match a 357 Magnum’s energy but wanted more capacity than 6 rounds. The G29 holds 10 or 15 rounds of 10mm Auto and if I load those magazines with a full power load, such as Buffalo Bore’s 180 grain JHP ammo, the advertised velocity for my barrel should produce around 1300 feet per second and about 675 foot pounds of energy. Those are impressive numbers! Would this round guarantee a 1 shot stop or knock down in a life threatening emergency? Not at all. Why? Too many variables.

Before we jump into the variables argument, lets take a look at humans. It’s been said that the average heavyweight boxer generates around 700 ft/lbs of energy in a punch. If you’ve seen much boxing, you know how many punches to the head some of these guys take and some even go on to have very long careers taking massive amounts of energy to the head. That’s not to say that getting hit with a right cross is the same as taking a bullet. Far from it, but what it can show us is that while both a punch from a heavyweight fighter and bullet fired from a firearm can deliver similar energy, obviously a punch doesn’t enter your body and have the potential to harm organs or your central nervous system….unless you find yourself fighting a Mortal Kombat Warrior, but that’s another article discussion.

Bottom line, energy is energy, but the potential to do damage or to stop a threat relies upon more than just energy and a true one stop shot or knock down power will rely more on shot placement than how much energy a bullet can deliver to a target. Using my G29 as an example, if a round was delivered to an arm or a leg, I’d wager that wouldn’t necessarily equate to a knock down, or a one shot stop. Granted I’ll concede I wouldn’t want to get hit with a 10mm in the leg, or a 22 for that matter, but I think the principle shows that shot placement will have more bearing on stopping a threat than brute force. This is where all of that range time will come in handy. The best way to stop a threat if your life is in danger is to hit vital organs and/or the central nervous system. It can be done with a 22 Long Rifle, sure, but you’d be better served to carry or protect your home with the round that you can put 5 shots in the 1o Ring time and time again. This is where quick follow up shots come into play, and this too is where the debate takes a turn. A good 9mm hollow point round will recoil less than my 10mm Auto. The 9mm will deliver roughly 50% to 65% of the energy of the 10mm, but follow up shots on target should be easier with the 9mm versus the 10mm. All of that brute force is moot if you can’t control it.

So we shouldn’t get so hung up on knock down power. I’m guilty of this myself. Next time you’re comparing two cartridges and trying to determine which is best, keep in mind George Foreman. Think about how many times he got punched in the head with more force than my 10mm Auto and he’s still selling things on TV and forming complete sentences. It’s not the force behind the bullet. It’s the marksman behind the trigger!

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Categories: General
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About Aaron McVay | View all posts by Aaron McVay

Born and raised Iowan. NRA Lifetime Member and firearm enthusiast since 6th grade. Aaron’s interest in responsible carry issues has increased over the years, and is a large supporter of…

Born and raised Iowan. NRA Lifetime Member and firearm enthusiast since 6th grade. Aaron’s interest in responsible carry issues has increased over the years, and is a large supporter of the 2nd amendment. Aaron is also a big 10mm Auto enthusiast and owns a Glock 29 Gen 4. One of his lifelong goals is to begin long distance target shooting out to and including 1000 yards. Aaron has a wife of 10 years, 3 kids and is an IT Consultant who specializes in Enterprise Content Management Solutions and process automation.

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