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if act as a ghost ring sight is good...What good does you a diopter sight on a submachinegun firing from an open bolt?
Imo both were average. The MP40 was/is a great weapon to shoot, but also still too expensive due to its telescopic bolt. The Sten is a little too radical in the "simplify for manufacturing" department and is uncomfortable to hold and unreliable. I held both and shot a semi auto replica of the MP-40.
PPS-43 probably takes the cake.
The Sten, it is my understanding, was based on a design the Germans rejected while the MP40 was a bit overengineered for what it was expected to do (a common problem with German Weapons).