Quoted:
Fast, easy, and cheap. Disposable and burnable too, if that gets your attention.
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This was the main part. The key. You could cut one out of plexiglass and bolt it together if you wanted too. If you needed one and it only had to last for a few dozen rounds, which is an extremely conservative estimate, it would work. An AR lower holds the pins and the upper, which isn't that hard of a job. If you wanted, you could make a lower out of wood. I don't know how long fire control would be right, but as far as strength goes, it would work. Just don't butt stroke anyone or use it as a bat and it would be a real firing gun.
As fair as injection molding being expensive, yeah it is if you are talking about something as big and complex as a CavArms lower. But, if you redesign the lower into small, puzzle-piece parts that have easily machined molds, you could do the parts on a home injection setup. Again not the best thing, but if you [i]NEEDED[/i] one, and it was the only way, it would do.
The Sten wasn't a great subgun, but it was cheap, and it did the job. That is why it was a success. If you have nothing, then almost anything will do. No, it wouldn't be as good as a machined forging, but it would do. If you free your mind and allow yourself to get away from some preconceptions, like standard pushpins, typical looking receivers, thinkness here, thiness here and just let yourself think of a design which just [i]WORKS[/i] and just plain does it with no frills, you can do that.