When someone mentions WWII weaponry, and especially sub-machine guns, the Thompson, MP40, and British Stirling seem to be the first ones mentioned—and rightly so—due to their battlefield prowess and efficiency.
However, there’s one firearm in particular that is both highly underrated and often forgotten: the Russian PPSh-41. As crude and reliable as other guns made by the Soviet Union in that time frame, the PPSh-41 relied on several features that no other sub-machine gun had: rate of fire, magazine capacity, and barrel length.
While most combatants in WWII opted to chamber their firearms in 9mm luger or .45 ACP, the Russians continued to use their proprietary 7.62×25 cartridge shared by the Tokarev pistol, TT-33.
The smaller .30 caliber round allowed the magazines for the PPSh to carry a considerable amount of ammo, giving a typical infantryman 35 rounds in a box magazine or 71 in a drum. With a rate of fire of almost 1,000 rounds per minute, the infantryman wielding it was a forced to be reckoned with. It was so respected that the Germans would repurpose these guns to fire their own 7.63×25 Mauser cartridges.
Although you can’t buy them in their select fire configuration, several companies have made semi-automatic clones of the PPSh in both its native caliber and .22LR. Check out the Excam PPSH in .22LR for more info.