The Whole Nine Yards of Kinetic Energy


“Poetry in motion” - the heavy bullet


Anything that has mass (in our case a hunting bullet) receives two extra properties namely momentum and kinetic energy once that mass is in motion. Both these properties have values that can be calculated. Momentum is calculated by multiplying the mass of the bullet with its velocity and it is expressed by way of how many pounds of weight could theoretically be moved by one foot every second by the bullet’s force that is acting upon the object that is struck.  A figure of typically 60-75 lb.ft/sec for an average, non-magnum .30 calibre is about normal and about 55-70 lb.ft/sec for a non-magnum 7mm calibre.  These low sounding figures are in fact quite adequate for good quality cup and core bullets to penetrate the shoulder and reach the heart in all big game when the heavy bullets in that calibre is used.  Clearly implicit in the term momentum there exist the concepts of force and direction - in other words momentum is a distinct vector quantity which is able to do work.


Momentum is the only motion-based property of a bullet that is useful when considering what it will do when it hits an object like the shoulder of an animal.  It determines how the bullet may (or may not) enter and penetrate the target and then maintain its ability for sustained penetration after the kinetic energy within the bullet had completed its disruptive action on the bullet’s shape and sectional density (SD). The lower adequate momentum figures for a 7mm bullet compared to a .30 calibre mentioned above is due to its higher sectional density at similar weights.  The required momentum to execute this work however is a VERY variable value after impact.  SD may have been drastically reduced by bullet expansion and / or loss of weight like the  Nosler Partition, so the static values of SD given by manufacturers can only be used as a guideline.  The practical issues around this will be closely considered later in the article.


Opposite to a bullet’s momentum which is a tangible, vector value that depicts its ability to do work, kinetic energy of a bullet is a very amorphous entity that has no direct bearing on what will happen to the object that the bullet will strike.  Being presented as a scalar value (joules - which is another way of saying degrees of temperature) It can at best be used to forecast what will happen to the carrier of the kinetic energy - in our case the bullet - upon striking the target.  The KE value of a bullet is basically calculated by dividing the bullet’s mass by two and then multiplying that figure by the square of the velocity of the bullet at the moment of striking the target. The figure is expressed in joules (or rather incorrectly and at best confusingly as ft.lbs in the American system).  Unlike momentum figures which indicate the bullet's relative ability to do work there exists no explanation how the scalar value depicting the amount of latent heat carried by the bullet due to to its kineses (rate of  movement) may effect the target.  Kinetic energy, once the kineses is reduced after impact is re-translated into mere heat and therefor can only have an effect on the carrier of the KE namely the bullet itself.


A very impressive, very large figure of kinetic energy value is obtained by squaring the already high figure in feet per second of a bullet's speed, and using that as the multiplier to give some value to half the weight of the bullet.  This four digit figure has no direction or rate of application, because not being a force of any kind KE is expressed as a scalar value and not as a vector  value.  It is at best a heat potential - not unlike electrical voltage is a potential. Like the 38,000 volts that flashes across the two electrodes of a vehicle’s spark plug which can not kill a man, kinetic energy can not kill either.  SOMETHING ELSE is necessary to be present at the same time which supplies the force that assist in the pushing of the bullet which then, like how a sword mechanically cuts the heart or other vital organ, killing a human or  animal.  In a bullet the mass of lead and copper and its momentum in a certain direction does this in a simple physical way.  In an electrical discharge the mass of the electrons is that force.  Like the 38,000 volts from a vehicle’s ignition coil which can not kill, no matter the level of kinetic energy a bullet has, unless there is a SUITABLE amount of mass present relative to the mass of the animal, there will be no direct killing ability.


The low 120 volts from a home power outlet, even though the potential is very low carries a huge mass of electrons containing huge momentum - the amperage - which is the force that causes the physiological disruption that kills because it can overcome the impedance (resistance) in the human target.  Neither high voltage nor high kinetic energy kills - the momentum that is present within the bullet supplies the force that works the bullet through whatever resistance it may encounter in order for the bullet to mechanically cut a hole through an animal’s heart - the surest and cleanest way of killing the beast.


Ammunition and bullet manufacturers for many years have been selling the concept that light-weight bullets at very high velocity contain such high kinetic energy and that this is enough to theoretically kill the game being shot.  When calculating mass x velocity into a momentum figure for fast  light weight bullets may show the necessary value for penetration into the heart of an animal.  However as a force  to push a bullet through bone and flesh a theoretical value of momentum can still not stand on its own as will be shown later - and the biggest threat to its ability to be of sustainable value is the high KE value relative to its weight;  as is discussed further on high KE causes rapid expansion and drag, maybe even dis-integration and weight loss - both which immediately reduce the bullets sectional density which causes  retained momentum and penetration to suffer.


As an example:  a .300 RUM 180gr Hornady Interlock bullet at 40 yards indeed has the same kinetic energy as a .458 WM  500gr bullet of the same design at the same distance.  What the hunter with the .300 RUM will see as the effect of high KE on his target when the bullet strikes the low shoulder of his Cape buffalo at 40 yards will without a doubt change the expression on his face. A combination of high kinetic energy and low bullet weight has been and continues to be the reason for varying levels of distress for many hunters and guides of hunters - whether they are after dangerous or non-dangerous game - not to talk about the distress for the animal that was hit. The physics which are the cause for these uncomfortable moments (you know, that sudden, very human reaction of wishing one was able to turn back time by just a few seconds…) following impact by a too lightweight bullet will be explained in the discussion below.


The effect of kinetic energy

When a bullet is slowed down the kinetic energy carried within it is converted into heat which softens the lead and assists the mechanical flattening of the exposed lead tip; this rapidly increases the frontal area of the bullet which then equally rapidly increases the drag force on the bullet and simultaneously seriously decreases its sectional density.  Lower SD and higher profile drag causes penetrating velocity and thus momentum  to be immediately reduced. In the worst case, should the mass of the bullet be too low relative to its KE value the heat released by the diminishing kineses assists a too large mechanical disruption of the bullet structure and may even completely destroy it, along with whatever sectional density had remained, preventing any further linear penetration.  Complete disintegration is regularly observed with 140gr 7mm and 150gr .308” bullets at impact velocities of 2,800 ft/sec or more into a large animal’s shoulder or on an outside rib.  Increasingly poorer penetration starts occurring at impact velocities above 2,600 ft/sec.


To repeat these laws of bullet impact:

  • The destructive effect of high kinetic energy on a bullet is mostly a factor of the bullet’s mass and to a lesser extent its construction. The lower the bullet mass for any value of KE the larger is the bullet’s explosive increase in frontal surface area and loss of little fragments at impact to even full destruction.

  • The moment sectional density is decreased in a mushroomed bullet the effectiveness of whatever value of momentum had remained is greatly reduced and this effectiveness may even be totally lost. While momentum is the force needed for penetration its biggest enemy is a high kinetic energy value which causes larger expansion, loss of retained sectional density and therefor penetrating velocity.  This means in order to maintain acceptable penetration a bullet with more mass in the same calibre may be needed even if that means lower impact velocity.  The heavier bullet better withstands the self-destructive effect of kinetic energy.


To revert to the example mentioned earlier:  A 180gr Hornady Interlock bullet from a .300 RUM at 40 yards has the same kinetic energy (5,200 joules) than a 500gr same type bullet from a .458 WM.  The 500gr bullet has about 150 lb.ft/sec, momentum force and that of the 180gr .300 RUM is only about 84 lb.ft/sec.  When rapidly slowed down by a buffalo’s shoulder, that massive KE value carried inside a mass of only 180 grains of lead and copper destroys the bullet.   The same K.E. release into the higher mass of the 500gr bullet merely causes it to expand to maybe 1.5 x calibre with good retained momentum and penetration and little weight loss.


Both have the same K.E. value of 5,200 joules.  So why does the 500 gr .458" bullet penetrate and the 180gr .308" bullet break up?  The 500gr at much lower velocity but same KE has sufficient retained momentum to penetrate skin, muscle and bone.  Also, the 500gr bullet has more mass that can withstand the attack from 5,200 joules of KE, so it maintains its mass and momentum but the 180gr bullet’s little mass gets destroyed by that same 5,200 joules.


Another example of retained sectional densities: consider the SD of a .50 calibre round ball (about 183gr) from my Kentucky long rifle launched at 1,700 ft/sec into wet pack. Now roll that 183gr .50"  round ball into a .308" bullet and load it in a .30-30 case and shoot it at the same velocity into the wetpack.  Which one will penetrate better?  Then, if we could, we shoot a British £2 coin, also weighing 180gr, but of 1.2" diameter flat side on into the same wet pack at the same velocity.   All have the same weight and the same momentum but clearly not the same sectional density.  That mind picture of the three projectiles is visible sectional density. Which one would you take to put into the shoulder of an elk to penetrate and reach the heart? 


So, despite the distinct focus on momentum in this dissertation it alone can never mean anything more than a point to work from if we want to forecast the potential ability of a bullet to kill and elk or a Cape buffalo or even a little springbok or pronghorn.  Momentum needs a good sustained sectional density figure to enable it to push the bullet into the mass of skin, muscle and bone in its way.  The moment a high K.E. value starts its destruction on a bullet’s front end, SD is destroyed and then a higher momentum force (more mass) must be available to keep pushing the bullet along.


Impact Momentum

Unlike kinetic energy which merely is a “potential” and not a force, momentum is a measureable force in a distinct direction. It is the driving force that pushes a bullet on ahead against whatever resistance it meets in order to reach the heart of an animal and kill it in the quickest and cleanest way that is humanly possible and "humanely" required.


So, when better penetration through the shoulder and / or less meat damage is required the cleverest way to go about it is to increase the bullet weight in the hunter’s preferred calibre. Even though velocity is reduced, kinetic energy stays the same but momentum is considerably enhanced. Look at the example of the .280 Remington on a big animal's shoulder:


Bullet Weight        Muzzle Velocity       Muzzle KE            Momentum            Penetration

139gr                        3,150 ft/sec.               3,063              62 lb.ft/sec              Poor. Break-up.

175gr                        2,780 ft/sec.               3,004 f./lb              70 lb.ft/sec              Very good.



In Summary -  again some general considerations

A hunting bullet for killing an animal for meat by definition is required to retain a minimum impact-momentum value in order to maintain penetration while its frontal area is increased during impact. This means that expanding must never be immediate and certainly not uncontained. Most certainly it must never be explosive like light weight Nosler Ballistic Tip bullets at magnum velocities are. The controlled, typically not more than 2x calibre expansion of only the tips of Peregrine or the Barnes TSX bullet make them the supreme projectile for hunting with particularly the magnum calibres and their associated high velocities.


However, even the Barnes TSX gets negatively acted upon by high K.E.  On 7mm bullets in the 140gr category, at impact velocities above 2 700 ft/sec the expanded petals fold right back onto the shank of the bullet and almost gets fused to the shank by the heat of the high KE, so there is zero expansion to be able to cut a larger than calibre wound channel (Chris Bekker, Gun Africa, 3rd edition).  So, above 2,700 ft/sec. impact velocity even the TSX bullet acts as a monolithic solid and zips right through the animal.


With magnum calibres and similarly hand loaded .270W / .280 Rem / 7x64 Brenneke it is imperative that the heaviest bullet is used and a shot through the low shoulder and through the heart is aimed for as this will ensure a kill. (in any case that particular shot placement should always be the hunter’s aim on big game). 


The reader must not read any attack on any cartridge calibre or bullet type in this discussion. I know there is no thinner skinned animal than the rifle owner who owns a certain calibre and who is used to aim for the flattest shooting bullet at the lightest-for-calibre weight and at the highest velocity and published kinetic energy figures. The facts represented in this missive are not based on bias against any calibre but are used to offer objective explanations as to why bullet behaviour is as it is. That “as it is” is based on many years of observation in the field and on research into the “why” bullet behaviour is at it is.


Questions like “How much energy is required to kill an elk” can not be answered as kinetic energy is not a factor in killing.  At best there can be honest advice regarding bullet weight and bullet design and impact velocity and retained bullet momentum that will ensure penetration through the low shoulder and heart. Examples can be offered - for instance:  A .308W with a good quality 180gr cup and core bullet like the Hornady Interlock is perfectly good enough to penetrate the low shoulder and the heart and may even exit the opposite side of an elk / kudu / wildebeest / gemsbok in the most pleasing, killing manner at 200 yards.


At the same time a .300 WM with a 150gr Nosler Ballistic Tip will certainly break up on the shoulder of these big animals at 160 yards and will not penetrate to reach the heart.   Should the hunter prefer the 6.5s with their excellent SD values he will be well advised to employ the heaviest bullets available for his particular rifle’s chamber and magazine specs when he is hunting big game in the elk / kudu category.


The design integrity of the bullet of course is a very important issue in retained sectional density and momentum. Nosler Ballistic tips have a bad reputation for this in the fast magnums in my experience as a guide in Colorado, but particularly so in Africa. Light weight Nosler Partitions at 2,800 ft/sec impact velocity have regularly displayed massive entrance wounds, partial or complete break-up and failure to reach the heart.


Almost shocking was to learn from a gun writer in South Africa (Chris Bekker in “Guns Africa”, Edition 3) of a 500gr Woodleigh soft point from a .470 Nitro Express that failed to penetrate an impala further than the rumen from a frontal shot into the chest. The bullet expanded to 2.65 x calibre and lost its core. Even a 180gr good quality cup and core bullet from a .308W would have completely penetrated that impala.


So a law that we can state from lots of field experience is: Any expansion beyond more than 2x calibre may seriously degrade a bullet’s sectional density, thereby limiting the ability of its momentum to overcome the excessive profile drag and therefor may prevent proper penetration.  Had that shot with the .470 NE been on a Cape buffalo and not an impala the outcome would have been exceedingly serious for the hunter. It turned out that Woodleigh conceded that there was a quality assurance problem with its large calibre bullets. One can only trust that they have subsequently fixed the problem and recalled the affected stock.


In my own experience in Colorado the past season complete separation between jacket and core occurred on a 150gr Remington CoreLokt bullet from a 7mm Rem Mag at 306 yards - the core left the jacket behind outside a mule deer’s stomach immediately after entering the skin between two ribs. The core stopped against an opposite rib without even damaging it, completely flattened. Fortunately it had damaged the liver on its way and the buck was tracked to where it lay down 300 yards further.  Had this been an elk or had the bullet first hit the outside rib we would have lost the animal.


High kinetic energy is a dangerous companion to light-weight bullets.

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