A key source of historical small-caliber, high-velocity thinking

While small caliber high velocity (SCHV) infantry projectiles are a result of a very long-standing trend, with velocities increasing from the time the atlatl let the human throw a spear harder and velocities steadily following a downward path as research and enabling technology converge, most readers probably haven’t read the key documents. Many of them are linked by Daniel Watters at The Gun Zone, and formal American SCHV research (which was being replicated by researchers overseas) goes back at least as far as the 1920s.

That said, a very key document in the development of the 5.56mm M16 from the NATO caliber AR-10 was this Ballistics Research Laboratory report by R. H. Kent, The Theory of the Motion of a Bullet about its Center of Gravity in Dense Media, with Applications to Bullet Design. It This version dates from 1957, but is a reprint of  1930 report. (Thanks to Dan Watters for the correction in the comments. We guess that’s what explains why the reference round is the .30 M1 cartridge, not the WWII vintage .30 M3 ball).

One set of interesting findings from the abstract:

It is pointed out that a large value of k may be obtained by the use of bullets having light noses and it is indicated that for a given muzzle energy there will be greater energy absorbed from light bullets than from heavy bullets. The theory is applied to the effect of the caliber on the amount of energy absorbed in the medium. Itisdiscoveredthat at short ranges, the amount of energy absorbed will tend to increase as the caliber is reduced.

The paper is not fully accessible to you unless you can read sheet music (equations) and are comfortable with differential calculus. But there are insights even an MBA can find. For example, after revealing the experimental result that bullets in animal tissue perform much like bullets in water, Kent compares the effect of medium density (air or water) on the projo’s “stabillity factor,” which is a variable influenced by bullet design and spin:

“s” is known as the stability factor of the projectile. In air, near the muzzle, its value is 2 for the Cal .30 M1 bullet, but in water, near the muzzle, its value will be only 1/400. Thus, so far as our computations are concerned, it may be neglected.

He goes on from there to demonstrate that the twist of the rifling has no material effect on the stability of the projectile in the denser medium, a conclusion that is at variance with, if nothing else, early AR-15 sales claims.

But there were definite advantages to the SCHV projectile. These conclusions explain a couple of them:

[I]t may be seen that for ranges of more than four inchea in water, the greatest energy is absorbed from the smallest bullet. If the bullet were to hit an object like a bone, the smallest bullet would show a still greater superiority as far as the amount of energy absorbed i s concerned.

From the preceding discussion, it may be seen that if the caliber of the infantry rifle is reduced, that no reduction in effectiveness at short ranges will be obtained, and that in fact at these ranges the stopping and shocking power will probably increase. At long ranges, the srnaller bullets will have lost more velocity than the larger bullets and will thus have a smaller energy absorption at long ranges. This characteristicofthesmallerbullet should prove advmtageous since, at mch ranges, it is probably desirable that the bullet wounds eh&l not be lethal.

Others, of course, were flatter trajectory at shorter ranges (which happen to be the most typical combat ranges), and reduced recoil.The ultimate conclusion of Kent’s report, stripped of its bodyguard of scientific cautions:

[C]onsiderable improvement in the effectiveness of the infantry weapon may be obtained by a reduction of the caliber below that which now exists….

And of course, he followed that up with a call for — what else? — more research!

This is one of the foundational documents of 20th Century weapons science, and if carefully read, you can see the genesis of the peculiarly base-heavy 5.45 Russian round, as well as our own 5.56.

Update

This post has been updated in two ways. A correction from Daniel E. Watters has been included in the text. And we have cleaned up some messy pronoun misuse that got by our layers and layers of editors.

4 thoughts on “A key source of historical small-caliber, high-velocity thinking

    1. Hognose Post author

      I don’t think I’ve ever gotten a post on this subject up without you correcting an error… thanks humbly. I’d cut you in on a share of the profits, but there aren’t any!

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  2. Nathaniel

    Please do note the bolded part of the paragraph:

    For the M1 bullet in water to have a stability factor as great as .1, would require that the linear velocity be reduced to value of about 1/7 of the muzzle velocity or 400 f/s, the spin being assumed constant. Such a low velocity is not attained until the projectil has reached a range of about 4000 yards, and thus for all ranges of practical importance, it may be seen that the twist of rifling will not have any appreciable effect on the motion of the bullet in a medium like water except in so far as the initial yaw of the bullet is dependent upon the stability factor in air.

    In other words, while the stability of a bullet rotating at 200,000 RPM (about what M193 spins at from a 1/12″ twist barrel) is virtually no worse in water than a bullet rotating at 340,000 RPM (for M193 from a 1/7″ twist barrel), the angle of attack of the bullets when they hit the target will be different, because the projectile fired from a 1/7″ twist barrel will be more stable in air and thus yaw less in flight. The angle of attack upon striking being an important factor in the terminal performance of a bullet, one should select a twist rate which gives the desirable characteristics.

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