"But submachine guns were really intended to be cheap and easily mass produced."
On that Suomi M31; pretty much that entire receiver (the torch cut piece) was machined from a single massive forging. They bored out the interior of the tube with some crazy stepped reamer, machined the sides of the lower and the tube profile, turned the endcap threads and barrel shroud lugs, riveted on the stock tang and the front/rear faces of the magwell, polished everything to the point no tooling marks or rivets are visible, then oven-hardened the steel to something like 60 Rockwell before bluing them. The bolts were around 70 Rockwell.
For what was basically a piece of pipe, a lump of steel, a barrel, and a spring, the Finns really threw everything at it. But, they ran fantastically, would hold 10" groups at 20 meters on full auto, and contributed greatly to the upper body strength of the Finnish fighting force
. To understand the appeal and history of the guns, you have to realize the Finns kicked the Russians' rears so decisively during the Winter War that the gun was cloned, the magazines directly copied, and the whole SMG concept thought to be the future of warfare for a solid 20 years by the entire planet*. Understanding the dynamics of that forgotten conflict may be interesting (I am ignorant of the details, other than it was SMG vs. bolt gun, plus a smattering of early LMGs on both sides, and that the Finns did a lot fewer idiotic mass-charges than the Russians)
TCB
*Maybe not the Japanese; did they ever make an SMG?