Infantry firearms calibres - a long history

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(This post was motivated by a somewhat heated exchange with someone who insisted to see a trend.)

Why did we end up with a calibre of 5.56 mm (and others with 5.45 or 5.82 mm)? The understanding of modern infantry firearms calibres requires in part an understanding of their history.

The typical European 18th century service long firearm was a musket. The calibres were astonishingly large, and the typical bullet was actually a spherical ball. Cartridges were introduced, typically paper with ball and blackpowder, and de-mixing of the blackpowder over time was an issue.

 

The famous British Brown Bess may serve as a representative of this era:

It was a smoothbore muzzleloader. The barrel bore was .75" (19 mm) and the typical ball fired was .69" (17.5 mm). The difference between bore and ball (windage) differed between countries, less made loading slower, but the shot dispersion was made smaller. The tolerances in producing these guns were such that substantial differences even in calibre were common (and were tolerable at the time). The heavy ball (about 550 grains / 35.5 g) was fired with a low muzzle velocity of about 1,000 fps (305 m/s); subsonic.

Brown Bess / Short Land Pattern Musket, 18th century (c) Antique Military Rifles

The invention of the Minié ball made the conversion from smoothbore musket to a much more accurate rifle possible without the slow reloading of earlier rifles: The bullet (cartridge) of the Minié ball was subcalibre during loading and thus easily loaded as was a subcalibre ball cartridge, but the gas pressure widened the soft lead bullet and pressed it into the grooves. This enabled spin stabilization of the bullet for a small dispersion and real sights and aiming at individual enemies became purposeful for line infantry. It makes little sense to spin stabilize a ball, though; the typical bullets for rifles are longer than the calibre. The Minié ball actually required this for its function.

The weight of such a bullet became almost prohibitive with the established calibres. It wasn't necessary to maintain the established calibres anyway, for the greater length of the bullet added to its penetration capability. The invention of pressed blackpowder grains made blackpowder more consistent and more potent and helped the rifles further.

The consequence was a reduction of the calibre.

A representative for the typical Minié ball rifle shall be the Springfield Model 1861:

Springfield Model 1861 rifle-musket

It was a rifled muzzleloader. The barrel bore was .58" (15 mm) and such was the bullet calibre after leaving the muzzle. The muzzle velocity of the approx. 510 grain (33 g) bullet was apparently about 960 fps (290 m/s) - both a bit less than the Brown Bess'.

The Springfield Model 1861 rifle-musket was already obsolete before its predecessor was introduced, of course. The Prussian Dreyse needle gun (in service 1848) had finally -after centuries of experimentation - delivered the first truly successful breechloading service rifle. Its only advance was its (initially) higher rate of fire, other specs weren't very different from the rifle-musket competition.

The superior French Chassepot rifle (in service 1867) shall instead represent the late age of blackpowder breechloading rifle:

Chassepot rifle, (c) PHGCOM

The calibre was already no more musket-like, but down to 11 mm. The bullet was down to 386 grains (25 g), but its muzzle velocity was up to about 1,350 fps (about 410 m/s) - supersonic. The bullet wasn't weaker than the others mentioned, but it was lighter. This was a purposeful adaption to the new quick fire capability. In theory (if the rifle works flawlessly), a riflemen could have consumed the previously normal maximum 60 rounds within four minutes of quick fire. The only sensible way to supply the rifleman with the ammunition to exploit the quick fire of his rifle was to use a lighter bullet. This saved both weight and money.

The long-ish shape was determined by the nature of the rifle, so a smaller calibre was the way to go. A lower density material would have been no good choice, as the very dense soft lead was very good for the grip in the grooves (internal ballistics), external ballistics (high sectional density) and for terminal ballistics (flattens in soft targets).

Then came the 1880's and quite scientific chemistry. The first practical 'smokeless' (actually less smoke) powders (actually grains) were introduced (more here). One of their consequences was that quick and automatic rapid fire became more practical because the smoke didn't impede aiming any more. Another was that higher muzzle velocities became achievable.

The former meant that repeating rifles became more important as service rifles in place of single loading rifles: Built-in magazines loaded with a clip became common for decades to come. This meant even higher practical rates of fire and the weight issue became even more pressing. What's more important was the gain in external ballistic performance; the muzzle velocities were approximately doubled.

The German (Mauser) Gewehr 98 may serve as a representative for this era:

Gewehr 98

It was the culmination of a series of troubled designs. Experiments with 5 mm (1892) and 6 mm bullets (1998 at the latest) were unsatisfactory, and bullets were not yet really understood from an engineering point of view. The 8x57 mm "Patrone 88" cartridge with its 14.7 g bullet yielded a muzzle velocity of 640 m/s. This bullet was disappointing. The high muzzle velocity and conventional bullet shape led to compression effects which damaged the barrel and caused too much wear of the grooves.

Research led to the adoption of a Spitzgeschoß* (spitzer bullet) of 9.8 g for a muzzle velocity of 895 m/s. The reduction in weight was intentional, but also a consequence of the new shape. The shape solved the internal ballistic issues and also offered much superior external ballistics. The new bullet design also led to a very different behaviour after impact (terminal ballistics).

This spitzer bullet revolution of the 1900's (developed about 1898, introduced in armies during the 1900's)  followed the initial 'smokeless powder' revolution of the 1890's which in turn followed the conversion to breechloading in most European armies during the 1860's and 1870's. It sure was an era of unprecedented 'progress'.

So again, a drop in bullet weight was accompanied by a jump in muzzle velocity, and this was again due to the technological progress in bullet design, again with minimal alterations to the weapon itself.

It is interesting to see that smaller calibres (8 mm was considered 'small calibre' by 1900) were already tested, but found wanting. Their higher muzzle velocities (of 1890's 5 and 6 mm bullets) were too troublesome until spitzer bullets were developed. Some 6.5 mm calibres such as 6.5x55 mm were used by small powers and Japan, though.

Several rifle calibres of about 1900 were successful and became well-established, not the least because of the huge stocks produced during the First World War: 7,92x57 mm, .303, 7.62x54mmR, .30-06.

These cartridges proved to be too powerful for a practical self-loading rifle at acceptable weight until a solution was found with the Garand rifle. Many prototypes blew up. A semi-automatic (self-loading) rifle didn't seem to be the solution of choice after the experiences with trench fighting and machinegun fire during the first World War anyway; a combination of a rifle with the full automatic short range firepower of the newly invented submachineguns was desired. This was of course even more troublesome than a self-loading rifle.

The way to go was to use an intermediate cartridge, and a solution was found during the 1930's within the usual calibre range (7.75 or 7.92 mm), but with a much shorter, lighter bullet and accordingly less propellant in a shorter case (a Swedish 7 mm light machinegun cartridge didn't succeed). A loss in long range firepower was considered acceptable given the experiences of the First World War. This time, the need to reduce the power of the cartridge drove the specs.

The German Sturmgewehr 44 family of machine carbines  / 'assault rifles' shall serve as a representative, albeit the more notorious AK-47/AKM fit in the same era:

Sturmgewehr 44

It kept the calibre of 7.92 mm calibre to facilitate quantity production with available tools and machines. The bullet weighed 8.1 g and left the muzzle at 685 m/s (late wartime cartridge specs). This was considerably less than the contemporary "sS" 'heavy' spitzer bullet in calibre 7,92x57 mm, which had replaced the original spitzer bullets for in the mid-30's and weighed 12.8 g.**

Last but not least; how did we end up in the 5.45 to 5.82 mm range?

As mentioned, the first assault rifle generation still used the established barrel bores; 7.92 or 7.62 mm, with muzzle velocities around 700 m/s; well below what can be had without excessive groove wear (= a bit more than 900 m/s). There was clearly some potential for improvement for the probability of hit against moving targets (deflection shooting) and against low silhouette targets at unknown distances (flat trajectory desirable).

Another issue was how all-automatic weapon infantry used the bullets; it chewed through truckloads of them. Suppressive fire had become feasible and was found to be tactically very, very useful and also rather relieving for the stressed infantryman. 99.99something per cent of the bullets do not hit anybody, but they frighten many. 

This practice places a greater emphasis on the ability to carry many cartridges into battle, and rather less on the what a hit actually does.

The result was that the developers went back to the 5...6 mm range of bullets, which had been discarded in the prototype stage two generations earlier. The very small bullets weigh very little:

5.45x39 mm (Warsaw Pact): typical bullet about 3.5 g

5.56x45 mm (NATO): typical bullet about 4.1 g

5.82x42 mm (China): typical bullet about 4.2 g

The original introduction of the original 5.56x45 mm M16s as service rifles was probably by chance, but the subsequent adoption of this small calibre range for assault rifles and light machineguns all over the world was not. The move from the original full calibre assault rifle generation to this small calibre generation was driven by the need to carry more bullets into battle at an acceptable weight and bulk. This in turn was driven by the gain in rapid firepower that came from the acceptance of less effective range.

Ironically, the readiness to accept the latter compromise was worn out over time and much of this generation of soldiers wants more range again, asking for a different compromise (or an unobtanium bullet design).

- - - - -

I initially made clear that I'm opposed to the view that the reduction of service rifle calibres is some very, very old trend. I consider it rather as a series of technological (and ultimately also tactical) steps forward which by chance happened to all go into the same direction; smaller bullets.

It's a poor idea to consider this as a trend, though. A belief in a trend may create an impression that this trend may go on, and that may be misleading. It's better to drop the simplistic concept of a trend here and go right after what really mattered; the technological steps and observations of their tactical consequences.

An example why this difference may matter: The next observation that matters may easily be that  we finally look at modern battlefield gunfighting from the perspective known to so many ragtag militias and low budget mercenaries: We may find the performance of our bullets in face of hard body armour as too unreliable, at least without rather exotic subcalibre bullet designs. The consequence could be a short-term ammunition solution (more subcalibre high penetration cartridges; "SLAP") followed by a return to a bigger calibre with bigger bullets which have an easier time penetrating hard body armour. Maybe we even end up with two calibres carried; one for suppression and one for penetration.

A belief in an ancient trend isn't going to help us understand or even anticipate developments, for sure. The current calibre range had already been fiddled with 120 years ago anyway.

S O

defence_and_freedom@gmx.de

*: The French introduced a different kind of spitzer bullet slightly earlier, but it wasn't the kind that became the dominant design: It was a bronze alloy bullet.

**: This bullet design improved the external ballistics (relevant only at long range), but primarily it lessened muzzle flash, muzzle bang and recoil issues of the original S-Patrone. There was also a late-WW2 7.92x57 mm duplex cartridge which had two bullets from the 7.92x33 mm calibre, potentially doubling the rate of fire of the already notoriously high-rpm MG42.

For German readers: Einige Infos entnommen aus "Die deutschen Militärgewehre  und Maschinenpistolen 1871-1945", H.D.Görz, 1994. Auch nützlich: "Waffen der Kabinettskriege", "Waffen der Revolutionskriege" und "Waffen der Einigungskriege" von Georg Ortenburg, 1986-1990

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