I was wondering what you wanted to do if you wanted to make a Sten and used an original length barrel, but an handmade reciver.? Thanks.
The barrel length issue aside (as I think we've got that covered now), also understand that the version of home-made receiver you create needs to be one that the BATFE has approved as true-enough semi-auto.
Most of the classic simple blow-back subguns are incredibly simple. They fire from the open bolt. Pull the trigger and the bolt flies forward, chambering a round and firing it almost simultaneously (sometimes not even quite in the order you'd expect!) and then the bolt flies back and does it again. All the trigger/sear mechanism does is get out of the way and stay down until you want to stop the fun.
To be a legal semi-auto version two things have to happen.
1) The firing mechanism must be designed that lets this thing fire from the closed bolt. That often means that a hammer and firing pin get introduced into a system where there were neither before. That requires a much more complex pile of parts completely different from almost anything in the gun before, including drilling the bolt out for a firing pin and fp retainer.
2) The receiver must be constructed so that there is no possible way that any of the original full auto parts -- usually including the bolt itself -- could ever be installed in it. Sometimes the tube inner diameter has to be too small for the old bolt. Sometimes there are blocking tabs welded inside the tube and the semi-auto bolt is milled out to slide past them.
It's a pretty big deal to backwards-engineer such a conversion. It certainly can be done, and there are kits that others have developed and gotten approved that you can use.
Just make sure you aren't coming up with something on your own that's too close to the original.
Just because your gun doesn't fire full-automatic does NOT mean the BATFE cannot consider it an illegal machine gun.