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Sorry, but you are mistaken.

What you are referring to as momentum is actually kinetic energy. Sectional density is, to put it in the vernacular, a mere shadow of momentum. Momentum is wholly about penetration as it is the force that must be overcome in order to stop the bullet.

Let me first address Sectional Density.

Two bullets, one a .284” 160 grain Hollow Point (hereafter HP), and one a .338” 200 grain “Partition” (hereafter Part) arrive at the target with identical kinetic energies - let's say for this little exercise 3000 foot-lbs. Using the 3000 ft-lb impact energy, (ke), the impact velocities are 3064 f/s (HP) and 2748 (Part). The sectional densities are .283 (RN) and .250 (Part). The higher sectional density of the 160 grain would suggest that it will penetrate farther than the .338. The experienced will think not.

Let’s consider the momentum of each bullet. The 160 has a momentum of 2.177 slug-feet (since many here will be disinclined to use metric units), and the 200 has a momentum of 2.440 slug-feet. The momentum values correctly predict not how much one will penetrate, but clearly which one will penetrate farther. One could argue (and many here probably will), that the 160 is an HP. I chose that bullet configuration, to make a point, but point design is immaterial unless you believe that a 160 grain 7mm will penetrate farther than a 200 grain .338 Partition when both have identical impact energies. In which case, there’s really not much further discussion necessary.

A factor no one ever talks about unfortunately, is "power". Power – in a physics sense - is the rate at which “work” (again in a physics sense) is done. “Work” being defined as force times distance. Units of power are: watts; horsepower (for example 33,000 foot-lbs per minute equivalent to 746 watts); joules per second, etc. “Power” equals work divided by time. If the “work” is in units of joules, and time is in units of seconds, “power” will be in watts.

Let me provide an example of how “power” – in the physics sense – explains the difference in observed “lethality” between round-nosed “penetrators” and large-meplatted hollow points. In this example, we’ll use bullets of the same caliber, but different design and weight. Weights will be 115 and 160 grains, and designs will be large-meplatted HP and RN. Once again, both arrive at the target with identical impact energies (kes), but different momenta, and sectional densities. Momenta and sectional densities predict that the 160 RN will penetrate farther than the 115 HP. Probably no argument there. However, when we observe a “bang flop” with the 115 vs a run 100 yds and then die with the 160, does “power” and “work” explain the different terminal responses? I think they do.

So as not to engage the argument of “through-and-through” (t-n-t) vs “keep–it-all-inside” (k-i-a-i), (which I couldn’t care less about – dead is dead as far as I’m concerned), we’ll say that the 160 travels exactly to the far side of the animal and lodges just “perfectly” beneath the skin. Therefore it has dropped all of its energy inside the animal – none “wasted” (as might be argued by those that advocate against t-n-t shots). The 115 on the other hand, has just penetrated into the chest cavity where it “exploded” thereby dropping all of ITS energy too. Because both bullets had identical impact energies, exactly the same amount of force was required to bring both bullets to rest; therefore the same amount of “work” was done by each bullet on the animal. HOWEVER, it is certainly NOT true that the same amount of “power” was developed in the animal by each pullet. For discussion’s sake let’s say that the 160 traveled 18 inches (45 cm) and the 115 traveled 6 inches (15 cm). Disregarding the impact velocities but acknowledging that the 115 had to be going faster in order to have the same impact energy as the 160, the 115 will come to a stop FASTER than the 160 does. Again disregarding their exact impact velocities, let’s just say that the 115 comes to rest in one-third the time that the 160 comes to rest. (This is certainly favoring the “work” done by the 160, but we’ll keep this “conservative”.) Referring you back to the equation for “power” - “work” per unit time - and remembering that the “work” done was EQUAL, it is mathematically certain that the “POWER” developed in the animal by the 115 is in fact THREE times the “power” developed by the 160.

Now do not misunderstand me here. I am not advocating for either t-n-t or k-i-a-i. Neither am I advocating for “kinetic energy uber alles”. I am simply pointing out a plausible explanation for the OBSERVED devastating terminal performance of light-weight bullets when compared to the OBSERVED terminal performance of slow, heavy RNs. Everyone should shoot what they like as far as I’m concerned.

Sectional density is a different matter. It was developed in an era when calculators and computers weren’t even a vague conception. It was a way to APPROXIMATE momentum when calculating momentum was “beyond” the average “Joe”. Furthermore, “gunwriters” (ptooey), and bullet manufacturers “picked up” the SD baton and “ran with it”…. WAY too far. SD’s limited value is not a position I thought up one day. Most serious ballisticians with even a passing interest in terminal performance realize (and so state) that SD is a weak substitute for momentum. Given the mathematical abilities of today’s shooters, and the tools available to them, the ease of calculating momentum renders SD obsolete – at best.

Don’t get me wrong. The original poster here wanted some info on “penetration”. I suggested that he would get more satisfaction in that search by using momentum. Youse guys wanna stick with SD… knock yourselves out. You’ll get no argument from me.

Paul

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