Tactical Skills for an Adventurous Life

The 1930 Report that Changed the World- Sort of

Development of the M-16 and it’s 5.56 millimeter cartridge was not by happenstance. The program came from a mountain of study and theory focused on the ballistics. Daniel Watters carefully documented that history in his 5.56 timeline hosted over at Loose Rounds. One of the earliest reports of significance came from Robert Kent of the Aberdeen Proving Ground in 1930.

Aberdeen first published The Kent Report, rather verbosely titled The Theory of The Motion of A Bullet About Its Center of Gravity In Dense Media, With Applications to Bullet Design, in 1930. They republished it in 1957 as interest in small caliber high velocity (SCHV) rifles resurfaced. This republished version is the one I’ve linked to.

I’ll get to the details in a moment, but I want to lay something out up front. Actual shot placement into something vital, like the heart or brain, is the fastest killer. Aside from that, there was tissue disruption. At this point in ballistic history, everyone assumed the primary method of tissue disruption was energy transfer.

In other words, the more effectively a moving bullet transferred its kinetic energy to the target, the more trauma it would cause.

J.S. Hatcher challenged this assertion only a few years later, but with a focus on handgun bullets.

Dr. Martin Fackler also challenged the kinetic energy model in 1980s and drove our focus to the fragmentation characteristics of bullets. Despite that, there is still value energy transfer model. The characteristics described by Kent regarding yaw behavior are the same ones that affect how reliably bullets fragment. So Kent still applies.

Ballistic Energy

Since a big part of this is about the energy carried by the projectile, let’s talk about that. If we go back to some Newtonian physics, we know that kinetic energy is a combination of mass and velocity.

formula for calculating kinetic energy

Using that, a 55gr bullet traveling at 3300 FPS has 1800 Joules of energy, or 1329 foot-pounds force (ft-lbs).

In comparison, a 147gr bullet moving at 2600 FPS has about 2990 Joules or 2205 ft-lbs.

At the same speed, the bullet with more mass has more kinetic energy. But increasing velocity boosts that energy faster than increasing mass. For example, we only have to increase the velocity of the 55gr bullet by 12.5%, to 4200 FPS, to match the kinetic energy of the 147gr bullet.

In comparison, we have to increase the mass of the 55gr bullet by about 63%, to 90gr, to match the energy, all things being equal. From this purely mathematical perspective increasing the one component by 12.5% is easier than increasing the other by 64%.

Of course, we know that it’s probably easier to increase the mass a bit than it is to push a bullet at 4200 FPS, but that’s besides the point.

The intent of this little tangent is to show you that if we left just this portion of the physics out there, it’s easy to see why everyone assumed the heavier bullet was better.

Energy Transfer

Where Kent’s report really comes in to play was building a model to show how effectively those bullets transferred their energy to a soft-tissue target. If the heavier bullet retains most of its 2205 ft-lbs of force, then its extra energy is moot.

The point here is that the lower initial kinetic energy of the bullet is less important than the total energy deposited into the target.

The “killer feature” of Kent’s research was that smaller bullets with light tips were more effective at transferring energy to a denser medium. This was especially true at closer ranges.

The reason for this comes down to the stability of the bullet. Light bullets with heavier bases require more stability, hence the faster twist rates found in barrels firing them. As the lighter bullet impacts the target, it’s faster to upset and yaw. The greater surface area of a yawing bullet exposed to the denser medium means more energy gets transferred.

r.h. kent ballistic formula for kinetic energy absorption
A snippet from Kent’s research

Kent goes on to plot the energy transfer of different weight bullets fired into water using fancy math. The results showed that the lighter bullets transfer slightly more total kinetic energy than the heavier bullets when the water depth was more than 4 inches.

This becomes important later on for military implications when you consider that the average human torso is 10” to 12” thick.

Most Important Element

Kent emphasized that the key factor for effectiveness was the degree of yaw the bullet already had upon impact. The greater this offset, the more rapidly it would turn inside the denser medium and dump its energy.

What’s interesting here is that mentions stability factors and how it affects yaw. But he goes out of his way to mention that the degree of yaw in tissue is independent of a rifle barrel twist rate for the most part.

If you’ve read my article on twist rates, you might recall that the stability factor for most rifle bullets is between 1.5 and 2.5. If a bullet was to avoid tumbling as it transitioned from air to dense tissue, it would need a stability factor of 800. Though, to be fair, the barrel may affect the amount of yaw the bullet has in flight, which affects the angle of attack as it hits the target.

There’s another interesting element to this since the primary driver of the bullet tumble is the amount of yaw at impact. With 5.56, the bullet exits with up to a 6-degree yaw as it leaves the barrel. As it flies, it gets more stable, reducing the amount of yaw. In other words, you’re more likely to get maximum effectiveness at closer ranges. That’s probably one of the reasons for the M-16’s fearsome wound channels found in enemy casualties upon its introduction.

Implications

So what are the actual implications of Kent’s work?

The study highlights three primary benefits, which you’ll immediately recognize later on in the M-16 development cycle:

At the end of the report, Kent says that looking at smaller calibers than the standard .30 cartridge in use is worth exploring.

These considerations indicate, probably, considerable improvement in effectiveness of the infantry weapon may be obtained by a reduction of the caliber below that which now exists, but a rational choice of the exact caliber would have to be based on a very extensive investigation.

Keep in mind that this report first came about in 1930. Its ramifications were simply too extreme for generations of decision makers raised on the Krag, Enfield 1917, and Springfield 1903.

Kents own report mentions that the gains to close range trajectory and effectiveness come at the cost of long-range performance. Since the infantry services still preached iron sight precision at 1000 yards, this was too much of a drawback.

In the end, the major takeaway from all of this is simple: lighter bullets of the right construction could be just as effective as the larger caliber rifles.

Over to You

So what do you think? Did anything about this report surprise you?

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Cutright
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Cutright

Very nice. This is well into the terrain of my eyes glazing over, but it helps build a better understanding of what’s really going on. Good read.

Sunshine Shooter
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I love this kind of stuff. The actual equation is a little dense for me at the moment (I haven’t stepped foot in a math class in a few years), but this is the kind of stuff I love. I’m amazed at how long ago people were considering the SCHV concept. I’m also even more dumbfounded as to how the .308 and M14 were developed in light of this work.

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