I just bought a parts kit MKIII STEN and I'd like to turn it into a semi-auto shooter. Anyone know what this would involve and how much it could cost?? Does anyone know of any web-based info out there I could use doing this?? Machining shouldn't be a problem cause I know a machinist.

Also, is there some sort of build letter I need from the ATF?? Thanks for any info. :)

Sorry to tell you this but it is not possible to do.
You will run afoul of two federal laws, one is that it would be a short-barrel rifle and thus an AOW type weapon the other even bigger problem is that it fires from an open bolt which the manufacture of was outlawed in either 86 or 94, I can't remember which.
Of course you could add a longer barrel and modify the action but that would be massive amounts of work.

Quoted: Of course you could add a longer barrel and modify the action but that would be massive amounts of work.

There are guys that sell the stuff to do that. One in FL who's name escapes me. You can also find 16 inch barrel for until you SBR the thing.

So it is quite possible to do, I just don't know much about it.

semiautosten.com

try www.catco.net , they used to sell the semi auto kits and if you e-mail them , i'm sure they will inform you of the legal issues involved . stens are fun guns to shoot , a fellow some time back mounted a red dot system on a select fire sten 2 and placed like second or so in a sub gun shoot . good luck , and let us know how your project goes .