STG is shorthand for "Sturm Gewehr" which literally translates to "Assualt Rifle". Since this was the first appearance of the word "Assualt Rifle" it was come to be known as the first assualt rifle in the world (simultaneously coining the phrase "assualt rifle").
As early as 1934 the need for a compact selective fire weapon was shown to be necessary but encountered opposition from staff officers and also from Adolf Hitler himself who preferred the infantryman to use the traditional weapon of the rifle as he had done during W.W.I. This bias hampered the development of this weapon up to the end of the war.
A prototype was not ready until 1940 due to problems with a suitable cartridge design and after several experiments with different size cartridges, it was decided to use the standard size rifle round with a shortened cartridge (kurz) and make the necessary adjustments to the rifle design. The cartridge was known as the 7.92mm Infanterie Kurz Patrone 7.92 x 33mm (Standard was 7.92 x 57mm).
The initial contract was given to C.G. Haenel who started work on a weapon known as Maschinenkarabiner (Mkb) with Hugo Schmeisser (of MP40 fame) being the lead engineer on this project. By 1941 a prototype was ready, basically a gas operated rifle produced from pressings which were subcontracted out to the steel fabrication firm Mertz Werke. An order was placed for 50 pre-production weapons was filled.
Preproduction orders were placed with both Haenel and Walther (who had also been working on semi-automatic rifle design G41 and G43) which included guidelines that had to be adhered to. These guidelines included providing a suitable fitting for the standard rifle bayonet, standard muzzle threads and the Haenel magazine these were to be adhered to by both companies.
Production of the weapon was however very poor and by 1943 only a few thousand had been produced. There were some distinct differences between the two designs firing mechanisms with the Walther model using an annular gas piston with a sleeve to connect the bolt (which also acted as a means of cocking the rifle). The Haenel design incorporated the much better conventional type of long stroke piston which drove back the bolt carrier, causing it to tip the rear bolt upwards out of it's locking recess.
The StG44 was by no means light in weight, but it was the
world's first assault rifle in that it had select fire capability, was
chambered for an intermediate cartridge.
After World War Two, the design of the STG44 escaped to Spain and was redubbed the CETME. After many years of developement and cold war hostility, the German firm Heckler & Koch purchased the licsense for the CETME and developed the G3 Battle Rifle to combat Soviet forces in what could have been the eastern european flashpoint. (Thank god the cold war ended)
The G3 Battle Rifle and HK G36K are the finest, most reliable and accurate Assualt Rifles in the world.
None of those weapons would be possible without the STG44.