Tam's written a time or two on how much skilled machine work was used in making firearms in the past compared to how a lot are made now. Reason I was reminded of that was that at the range this morning I was distracted by a couple of bursts of automatic fire. Peeking around the partitions revealed a man firing an honest-to-Deity Thompson subgun. 1943 manufacture.
Yes, he let me fondle it, and put ten rounds through*. If I could afford the regular conversion of that much money into noise, I'd buy one. But to the point at hand: in the past I've had the opportunity to fire a Uzi and a couple of Ingram subguns, one of the latter in .45. Compared to them, the Tommy is heavy. That receiver is one big hunk of machined steel, making for the heart of a heavy firearm.
And that translates to money; big steel blank, lots of machining to turn it into a receiver, which involves a skilled machinist and machine time and cutters. Thinking of making this sucker by the many-thousands during a war makes the adoption of the M3 very understandable.