Quoted:
CATCo was selling a semi auto parts set and receiver for $300.00 check the new issue of Shotgun News page 200. It appears that the gun would still be a SBR.
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The Catco kits come with a sleeve or barrel shroud that you weld on over the stock barrel to create a legal length barrel (16"). The instructions plainly state that to attach the original length barrel is a violation of federal law.
Catco also sold at one time a new 16" barrel to use with these kits. Haven't seen any around lately though.
Also, the Tapco parts kits are Mark III and the Catco kits are Mark II design.
The Catco kit consists of a new receiver of a smaller inside diameter than the original, so you can't fit the original STEN bolt into it. A new bolt is included. It is of the inertial firing pin design and fires from a closed bolt, as opposed to an open bolt, which would be illegal for the majority of us to own. The barrel extension sleeve and various other parts are included. You would need a surplus MkII kit to complete this weapon. All legal. Even in California.
But if really wanted to, you could put a STEN together with a parts kit, some tubing, a welder and assorted hand tools in your garage.
Cheap and very illegal.