A modern trend in submachine guns is to use a hammer or striker fired mechanism to allow firing from a closed bolt for increased accuracy when firing single shots. These submachine guns don't seem to have a bolt which is noticeably heavier than ones which fire from an open bolt with a fixed firing pin.One day I'll beg, buy borrow or steal a slow motion camera and take the API in subguns myth on. Interestingly, Turpin is convinced his design API's, but I suspect that if I set up my STEN Mk.II and fire it with both dummies and live rounds, we won't be able to witness the alleged phenomenon.
Although saying that, some of the RUAG ammo has soft, uncrimped primers which get utterly munched (tested in both a STEN and MP40) which does suggest they're igniting before they're fully supported. I can't find the photos back, but I posted them on FB a while back.
For that matter, the American civilian recreational market has a fair number of conversions of submachine guns from open bolt to closed bolt so they can sell them as semi-automatic weapons and not need to be registered as "machine guns" under American law. From the limited amount that I have seen, this does not require increasing the bolt mass or spring.
So far as I am aware, the main reason that many submachine guns, especially older ones, fire from an open bolt with a fixed firing pin is because it's cheaper to build them that way.
This would make an interesting "myth-busting" episode, but I'm not sure how you could test it using an unmodified Sten.