Belts are because on fully automatic ... well like 20 shot six-shooter revolvers ... a regular magazine empties VERY fast, it's just a continuous crashing sound, films are not realistic.
Machine Gun: Sterling MK4 SMG - The Firearm Blog
Sterling was supposed to replace the 1940 STEN in 1944, but maybe not common till 1953.
The STEN and sterling are very light ( I thought Sterling can remove fingertips, though maybe it was the STEN gun that did that). Apparently a range of baby strollers (2004?) inc. one called a Sterling were recalled due to a tendency to do the same
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Both use 9mm ammo.
A modified Sterling is a Starwars Stormtrooper prop.
Used 9mm ammo in 1940s (don't know how compatible with Russian Makarov 9mm pistol ammo, actually I think there are 3 flavours). The Bren in comparison is used on ground with legs, has interchangeable barrel (instead of water cooling) and uses ammo more like a .303 rifle.
Sten - Wikipedia
The 1940 equivalent of a rapid load cheap crossbow, poor range, maybe 100m.
Sterling submachine gun - Wikipedia
Carry a lot of magazines for your machine gun.