Amazing what you can find on the internet.the STEN was considered perfectly fit for service. Once the war was out Of the way, the Army ran a Trial o’ truth with some new SMG contenders to replace the STEN. The verdict? Yes, the Patchett that had been in limited service since 1944 was better, but not so stellarly so it so it warranted the dumping of the STEN for it.
Most fail to feeds with a STEN were down to, misaligned mag lips, there was a gauge to check them, loading the full 30 rounds into the magazine, this put the spring under very high tension (Hence the loading tool) and in a less than good magazine, the stack stuck and .... click, fugigit! 28 and you were fine. And last but not least, despite the instruction manuals and instructors telling people endlessly, don’t hold it by the magazine, They did, And it could pull the magazine out of alignment and yet again..... click! Fuggit!
the MP40 with exactly the same single stack magazine design had exactly the same issues with fail to feeds - but as it’s German, no one mentions that it was less than perfect too.
Some of it might even be gen.