The Physics Behind Bullets

How bullets kill is actually explained by pretty simple physics, yet it’s a phenomenon that few hunters understand.

One of my favourite hunting stories is that of my musk ox hunt in the tiny Inuit community of Paulatuk. It took place nearly 20 years ago, and I was shooting what was, at the time, a fairly state-of-the-art, in-line muzzleloader.

Back then, 209 primers were the sole domain of shotguns and I was relying on a No. 11 cap for ignition. I was shooting a 240-grain XTP in a plastic sabot, over 100 grains of loose Pyrodex, and I was confident out to 150 yards, even on a critter the size of a musk ox.

But, the cold weather really played havoc with the rifle and after three days of constant -40 temperatures, the muzzleloader let me down.

We’d spotted a small heard of bulls on a distant ridge and made a long stalk, utilizing small dips and folds in treeless terrain. Amazingly, we made it to 75 yards undetected. After looking the bulls over closely, I settled the crosshairs on the largest. It was a slam dunk, or so I thought. As my finger tightened up on the trigger, a huge cloud of white smoke bellowed from the muzzleloader, but the lack of recoil was my first indication that something had gone wrong. We watched as the .45 calibre bullet arced through the air and came harmlessly to rest 50 yards short of the bull. Obviously the extreme cold had prevented the plastic sabot from creating a gas seal and the bullet sailed harmlessly out of the rifle at only a couple hundred feet per second.

I looked to my right and David, my guide, looked perplexed. This was the first muzzleloader he’d ever seen and despite all my reassurance that I could get the job done at 150 yards, he’d just watched the bullet sail 25 yards and tumble across the snow. He looked me squarely in the eye and said, “I think we need to get closer.”

His comment brought a smile to my face, even as I frantically worked to reload the muzzleloader. Despite protests from David, I readied for a second shot. This time, the sharp recoil was welcome confirmation that everything had gone according to plan. The big bull was soon down on the ground and I thought back to David’s comment about getting closer and smiled again. I remember having those exact thoughts as a young hunter: if the bullet could just somehow make contact with the animal, it would kill it. The truth was, no matter how close I’d have been with that first shot, the bullet would have just bounced harmlessly off the musk ox. It carried no magic powers with it. If it didn’t make a hole that caused the animal to bleed out or the central nervous system to be shut down, the bull was not going to die.

How bullets kill is actually explained by pretty simple physics, yet it’s a phenomenon that few hunters understand.

Many hunters feel that bullets somehow magically release a huge amount of energy and that energy is what kills the animal. Some go so far as comparing this to an explosion or a shock wave  emanating throughout the body that results in catastrophic damage. Unless you are using explosive projectiles, killing is still done by nothing but mechanical means. So what role does energy play and why do so many hunters believe that it, by itself, is a killing force?

The answer to the second question is pretty easy. The myth of the energy transfer can be traced back to John “Pondoro” Taylor, the inventor of the Taylor KO (Knock Out) Factor. It was a fairly simple formula that determined how much energy was required to put down a brain-shot elephant. And to Taylor’s credit, he did have a lot of experience shooting elephants, both legally and illegally, and during his time bullet selection was very limited so energy could be a fairly reliable guide as to how much was required to allow a bullet to knock out an elephant. But, as often happens, Taylor’s KO factor became the definitive guide for many when discussing the amount of energy required to kill all types of animals, with a variety of shot placements. While Taylor is often mocked for his theory, in truth, it likely was fairly accurate for determining how to knock out an elephant with the bullets at the time. He never intended a more universal use.

Let’s put one myth to rest right away: energy does not kill. There is no way to predict how much energy is required to put down any big game animal. There are just way too many variables that come into play, not limited to bullet construction and shot placement.

What energy does is allows a bullet to do its work. This includes expanding and penetration. Without energy, the bullet cannot do its work, but there is no deadly energy transfer, dump or shock wave . There is, quite simply, a solid, expanding mass passing through flesh and bone. Without penetration, there is no death – save for the most unlikely scenarios that don’t really bear discussion here. We are talking about shooting a deer or moose or elk in the vitals. Without a hole, nothing dies.

Different bullets, due to different construction and materials, require different amounts of energy to do their work properly.

Let’s look at what a bullet actually does.

Upon contact with the animal, it pierces the hide and begins to expand. Typically, the higher the velocity, the more rapid the expansion.

As the bullet passes through tissue, it pushes that tissue out of the way, creating a hole or permanent wound channel that is larger than the expanded diameter of the bullet. As this is not a sharp, cutting projectile like a broadhead or knife, obviously tearing of the tissue will occur as the bullet passes through, create the larger wound channel. A temporary wound channel is also created as fluid in the tissue is displaced by the bullet pushing through. This is what is often referred to as a shock wave, but in truth it is more accurately described as hydraulic shock. This fluid being displaced can cause some severe bruising in the surrounding tissue and when it occurs close to the central nervous system, it can even cause a temporary disruption of it. Think of it as similar to dropping a stone in the water.

Where many people’s understanding of energy fails, though, is their thought that this wave emanating out from the bullet is somehow a transfer from the bullet into a killing force. It’s not. It can temporarily disable an animal at best, other than in some extreme circumstances.

The old debate about whether you prefer a bullet to pass through or stay in the animal really accentuates how much stock some people put in the energy dump myth. Whether a bullet passes through or not has a lot to do with the range the animal is shot at and where it is hit and there is no way to accurately say that all will or won’t, when talking big game bullets. Certainly some have a propensity to pass through and others don’t, but that can all be traced back to bullet construction and, more importantly, the amount of weight they shed as they pass through tissue.

Bullets that retain weight well are more likely to pass through and those that don’t retain weight well will slow down faster and are less likely to pass through. But, in some people’s minds, bullets that don’t pass through somehow dump this huge amount of energy and are more deadly than those that don’t.


They don’t emit some deadly death ray of energy, they just fragment into small pieces and the more they fragment, the more that penetration is reduced. The energy of the main mass is lost to the particles breaking off, not turned into some deadly force.

Often, damage from these more frangible bullets is more pronounced, but not because of an energy dump, but rather just from the fragments breaking off the bullet and migrating outward from the channel created by the main body of the bullet. Varmint bullets that literally blow into pieces shortly after contact are an extreme example of this. They do this to prevent exit holes and excessive fur damage, but penetration is severely compromised. They make poor choices for big game, yet for those that buy into the deadly energy dump, they would seem ideal.

The ideal bullet for big game is one that expands well and penetrates well. Whether it passes through or remains in the animal is meaningless if it does the other two things well.

Despite simple physics explaining energy and dispelling the energy dump school of thought, manufacturers and some writers continue to propagate these myths. Read through the pages of any outdoor magazine and you’ll see ads about the amount of energy some new cartridges produce or the hard-hitting energy of a magic bullet. I guess it sells new guns and bullets, and I guess it really isn’t a lie, but more a manipulation of the truth. But the fact is, energy does not kill – it just allows the bullet to do its work so it can kill by putting a hole in vital organs and blood-rich tissue. Yes, the bullet requires sufficient energy to accomplish this, but the death is not a direct result of the energy.

So what does all this mean to the average hunter? Not a lot really. With modern cartridges and modern bullets, death is pretty well assured within average hunting ranges, with any of them, if the shot is placed well.

The only real consideration with energy is that there is enough on impact for the bullet to expand and penetrate. Different bullets have different requirements to accomplish this. I know for sure that no matter how close I was to my musk ox on the first shot, there was not sufficient energy for the bullet to do its work and definitely energy alone was not going to kill it, despite the thoughts of some that if that bullet could just make contact that energy would do the rest.

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