The Sten mag was supposed to be compatible with the Lanchester, an MP-28 copy and that was supposed to be compatible with the MP-34/38/40. They were double-row, single feed mags that relied on the spring pressure to centre the top round in the magazine. The modern double-row, double-feed magazine was (ISTR) invented around 1935 by Dieudonné Saive (later of THAT rifle fame) to fit 13 rounds into the grip of the new Browning Hi-Power.Indeed Mp38/40, Stench series, MP18I, 28I, Lanchester, M3 series Greasegun all have the same mag fault in their designs
So not just german crap but UK and US crap. face it the Wops had a better SMG
no amount of ruffling the times will fix that fact
The Sten magazine design problem was added to by the fact that the magazines were made by every metal-bashing workshop, tolerances were loose and there was no quality control. If you sorted out a set of reliable magazines, it was as good as any of its contemporaries but was liable to surprise unwary new users with its magazine unreliability. .