“The idea of a light infantry rifle capable of automatic fire can trace some of its legacy to the SG44, but it was probably inevitable.”
Trace it to the BAR.
But that was WWI so that won’t do.
“...Trace it to the BAR...” [jeffersondem, post 35]
John M Browning’s automatic rifle - sometimes termed the “machine rifle” - played no part in assault rifle development.
It was a finished design, on the shelf in 1910 (see _The Devil’s Paintbrush_ by Dolf L Goldsmith). The American military wasn’t interested.
Though dubbed a rifle, the BAR was much closer to a light machine gun. It was employed as a crew served weapon.
After they were defeated in the First World War, the German military establishment sat down and rethought many fundamental concepts of ground combat.
Extensive study of Western Front engagements revealed that infantry rarely engaged adversaries beyond 300 meters (330 yards); they may have fired, but by that distance chances of scoring hits shrank to a few percent.
Pondering the data, ordnance engineers reasoned that the infantry rifles of the day were overbuilt and overpowered for any missions infantry could be called on to do: most of them could cause lethal wounds at ranges beyond 2000 yards (1850 meters). They recommended that a cartridge with an effective range of 400-500 meters would do an adequate job. Great savings in cost, materiel, and weight could be realized. Individual troops could carry more ammunition.
And controllable full-auto fire from a rifle-size weapon might be possible - something no designer had achieved so far, utilizing only “full power” (30-06, 303 British, 7.92 Mauser, 7.62x54R Russian) cartridges fired from weapons the size and weight of standard rifles.
So the Germans developed the 7.92x33mm Kurz cartridge, weighing 1/3 less than 7.92x57mm Mauser, firing a bullet of 124 gr to about 2250 ft/sec, generating some 1390 foot-pounds of kinetic energy (less than half that of the full power 7.92x57mm Mauser, several times that of the 9x19mm Parabellum pistol and submachine gun cartridge).
Several prototype selective fire rifles were developed to fire the new round. Haenel’s design (from Hugo Schmeisser’s design team) was selected for mass production - over Adolf Hitler’s initial objections.
Much ink has been spilled, speculating over the choice of caliber. Simplest explanation was proposed by the late John Weeks: German gunmakers were already making many rifles of 7.92mm bore size, and ammunition manufacturers were set up to make millions of 7.92mm (0.323 inch) diameter bullets.
The same rationale has been attributed to the USSR, in adopting the 7.62x39mm cartridge. It uses bullets of 0.310 inch diameter. These have long been common in Russian (and later Soviet) military small arms, fired from many cartridges: 7.62x54R o1891g, 7.62x38mmR Nagant Revolver, 7.62x25mm Tokarev. Millions upon millions of rounds.