Below is a photo I found on the web. It is not mine:
The Sterling series of submachinegun was used by Great Britain after it retired the STEN gun. It is a side magazine fed firearm which accepts 34 round curved magazines.
Its mostly known as being the base prop of a Stormtrooper's rifle from the Star Wars series. George Lucas had a thing for cutting up and welding stuff to WWII firearms for props.
Parts kits for these old SMG's can still be found. Although they are not cheap. Often starting around $400. Magazines are still available around $20ea. There are some resources out there to help building a semi-auto.
Basically the parts kit is converted to fire semi-automatic only from a closed bolt. The receiver and bolt used much be modified in such a way that full auto components cannot be used. a 16+" barrel is also requires. A common substitute being an UZI carbine barrel.
The build itself is a "tube gun". What makes this built attractive is that it was made under the Imperial measurement system. So replacement tubing from 4130 would not be too hard to get. I was thinking that instead of ending the shroud like shown in the picture above, using a longer tube and extending it to the end of the carbine. This would also increase the sight radius, and a sight expense in weight.
This is also a TIG welding dependent build.
I am guessing the price tag on this puppy won't be cheap. I am guestimating that it would end up costing around $1k to make. Perhaps it would be possible to shave a hundred bucks or so if they were manufactured in a larger production lot.
There is a used one on Gunbroker (with no picture)
listed with a start bid of $1600. No reserve.