Introduction
First, I need to define what I mean by an "assault rifle", as there are various definitions around. The one I use is:
"A standard military rifle, capable of controlled, fully-automatic fire from the shoulder, with an effective range of at least 300 metres".
This has some clear implications for the ammunition such weapons are chambered for. First, it excludes all weapons designed around pistol cartridges (i.e. machine pistols and sub-machine guns - SMGs) as they only generate around 500 joules muzzle energy and cannot meet the range requirement. Second, it excludes the traditional "full power" military rifle/MG cartridges such as the .303", the .30-06, the 7.92x57, the 7.62x54R and the 7.62x51 NATO (typically firing 10-12g bullets at 750-850 m/s and developing around 3,000-4,000 joules), as these are so powerful that their recoil is virtually uncontrollable in fully-automatic rifle fire from the shoulder.
Pistols such as the Mauser C96 (7.63x25) and P08 Luger (9x19 Parabellum) produced carbine derivatives with detachable stocks, usually only capable of semi-automatic fire but a few with a burst-fire option. These were relatively expensive to make, however, so the future in short-range automatics lay with the much simpler API blowback SMG. The first of these in service (if you discount the curious twin-barrel Villar Perosa) was the Bergman MP18 in 9x19, which was the ancestor of the MP 38/40, the Sten Gun, the PPSh and so on. An honourable mention also to the Thompson, developed separately in the USA from 1916 onwards for their .45 Auto cartridge. An oddity was the Pedersen Device of 1918, which replaced the bolt in the US Springfield Rifle with a semi-automatic mechanism to fire small .30 cal (7.62x20) pistol-type rounds developing less than 400 joules; it was never used in anger.
Attempts to improve the power and range of the SMGs, such as the development of the .45 Remington-Thomson in the experimental Model 1923 Thompson SMG (which used a very powerful loading developing almost three times the muzzle energy of the .45 Auto) and the use of the 9x25 Mauser round in the 1930s Solothurn S1-100 and Hungarian Kiraly 39M and 43M SMGs (which saw some service), did not catch on. There is a limit to the degree to which the performance of such weapons can be increased as their large-calibre, relatively light and round-nosed bullets lose velocity quickly. Also, the basic API blowback system used by most SMGs is not suited to high-powered ammunition, although the Kiraly and Thompson M1923 had more sophisticated mechanisms.
