The name of the game to the military is lethality - how good is a rifle for killing? The answer, except when explosive projectiles are involved, is in momentum of the projectile, which is bullet mass times bullet velocity.
With early black powder, there was not enough pressure generated to get high velocity, so the choice was to use a heavy (thus large diameter) bullet. Bullet diameters of 3/4 inch (e.g., the Brown Bess) were common.
As powders improved, higher velocity could be achieved and lighter bullets became as lethal as the earlier big bullets. By the U.S. (un)Civil War, the standard caliber was .58 or thereabouts. This shortly went down to about .45 caliber or its metric equivalent of about 11mm. Around 1900, smokeless powder, with its higher pressure, allowed for bullets as small as 6mm/.24 caliber to be considered lethal enough if high enough velocity could be achieved.
Many armed forces, including the U.S. Navy, purchased 6mm and 6.5mm rifles, mostly because of weight savings and in many cases because their soldiers were of small stature. But those cartridges were not able to gain enough velocity with the powders of the day to achieve the desired lethality, and most of the armies that had gone to 6.5 and smaller went back up the scale, or tried to, to a bullet of around .30/8mm.
That turned out to be close to the ideal. Adequate velocity and bullet weight for good lethality and the penetration of defenses like logs and of that new invention, the automobile.
Arising out of the last days of WWII, there then came a demand for faster firing weapons, selective fire rifles which were given the collective name "assault rifles" (from the German "Sturmgewehr"). But these presented a problem when full power ammunition (as then understood) was tried. The recoil was excessive and the rifles were simply uncontrollable in full auto fire.
Given the alternative of being restricted to semi-automatic fire or again reducing the rifle caliber or power, armies chose the latter approach. Today, most military rifles are chambered for one of two medium power cartridges, the 7.62x39 or the 5.56 NATO. But the same problem faced by (among others) the Japanese and the Italians, once again rears its head. The light, low recoil cartridge, mainly the 5.56, has proven lethal enough as a pure anti-personnel weapon, but its light bullet fails in penetration of defenses. So, back to the future with something like the 6.8? Who knows, but it all sounds familiar somehow.
Jim
