Why The Army's Next Round HAS to Be Light – In Just One Simple Example

Nathaniel F
by Nathaniel F

We are at a crossroads in small arms development. Demands for improved weapon effectiveness have reached their apex. At the same time, the soldier’s burden has grown into a crisis so pressing even the Army Chief of Staff has acknowledged it in testimony to Congress. Soon the next ammunition configuration will be decided, as new technologies open the door for a rethinking of the infantry’s most basic weapons.

Whatever round they choose, it has to be light. Here’s why, in one simple example.

Since both the load of the soldier and round effectiveness are serious concerns for the next small arms configuration, perhaps the right answer is to split the difference and go with something directly in the middle. However, “in the middle” probably doesn’t mean what you think. I’ll explain.

Let’s assume two rounds: One weighs 10 grams, while the other weighs 20. Since the first round is half the weight of the second, a soldier can carry twice as much per kilogram. Let’s go ahead and express that mathematically:

1000 g / 10 g = 100

1000 g / 20 g = 50

If we want something “in the middle”, we need a round that allows the soldier to carry more rounds than the 20 g round, but we decide it’s probably OK if he can’t carry as many as the 10 g round. 75 rounds is right in the middle of 100 and 50, and we determine that would be satisfactory. Simple intuition tells us this round needs to weigh 15 grams to give us that combat load:

1000 g / 15 g = 66.66… !!!

That’s right, splitting the difference does not give us a combat load in between the two rounds, it gives us a combat load twice as close to the 20 g round as the 10 gram round! This case illustrates that our natural intuition with numbers is wrong, that in fact the halfway point is somewhere else:

1000 g / 75 = 13.33… g

So, paradoxically, the midway point for a given combat load between the two rounds is twice as close to the lighter round as the heavier!

To drive the point home, we’ll repeat the example with the masses of two real rounds: 5.56mm NATO and 7.62mm NATO. 5.56mm weighs about 12 grams per shot; 7.62mm about 24 grams. Keeping in mind that the average of 12 and 24 is 18, let’s see how the math shakes out:

1000 g / 12 g = 83.33…

1000 g / 24 g = 41.66…

1000 g / (83.33… + 41.66…)/2 = 16 grams

It’s counter-intuitive, but it’s simple math. The next round has to be light, because the halfway point isn’t where you think it is.

(For the math geeks in the room, you have already figured out that it is the harmonic mean you need to use for this problem, not the arithmetic mean. Since this is a family-friendly site, I omitted these terms to keep things understandable for everyone.)

Nathaniel F
Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.

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  • Dick Hertz Dick Hertz on Aug 18, 2017

    Imagine how great it would be if it were commonly understood that in modern small-unit infantry "fire and movement" tactics, the rifle is very nearly irrelevant and almost all the ammunition expended goes through belt-fed weapons of varying calibers.

    On the attack, the riflemen form a maneuver element to carry out assault of enemy positions with grenades and bayonets under cover of fire from belt-fed weapons. On the defense, the riflemen are there to keep the SAW and LMG crews from being outflanked or overrun.

    Krags and Trapdoor Springfields are more than adequate for the purpose of the infantry rifle--and might even have some advantages over modern ultralight short-barrel 5.56mm carbines in places like Afghanistan, due to politically motivated Rules of Engagement mandating that precision long-range rifle fire must do jobs vastly better done by heavy artillery and ARCLIGHT.

    So, why don't we talk about proper cartridges and designs for squad automatic weapons and light machine guns, which are the infantry platoon and company's REAL teeth?

    And when we talk about battalion level actions, the mission of the infantry battalion has been, for more than a century, to seize an enemy unit and hold it in place so that brigade and divisional heavy artillery--the REAL killers on the battlefield going back to before World War I--can rain death on their heads?

  • Scaatylobo Scaatylobo on Aug 19, 2017

    I still believe that if it were not for the NIH syndrome [ not invented here ].
    We would have had about 30 years [ or more ] invested in making the 7.62 X 39 into an AMAZING round that would fill in all the voids .
    BUT since that is never going to happen,we will hash and rehash the same old B.S. about the 2 same rounds.

    • See 2 previous
    • Scaatylobo Scaatylobo on Aug 20, 2017

      @Nathaniel F. I know my limitations,and I am a muzzle freaking scientist.
      So I wont pretend that I could guess what the past FIFTY YEARS of investing in the intel into the 5.56 would have meant for the 7.62 X 39.
      But all the materials and all the powders and all the different bullet weights = might have been a HUGE difference in what they issue now.
      In fact I will go so far as to say that IF [ big if ] "they" had put all that money and intel into the 'other' round,we might not even be discussing the 7.62 X 51 at all.