If you just want a 16" semi bbl threaded, I used ADCO before (for an M-16A1 bbl). Might give them a try. I was happy with the service (good work and fast turn-around) If you're talking about a full auto MK-IV bbl, I don't know. Just get (or convert to) a MK-V http://www.adcofirearms.com/shopservices/
I have tried to reach SAMACHINE for their barrel, to no avail. However, I am glad I did not reach them because I read that in order for their barrel to work, you have to ream out your existing barrel access hole on the end of the receiver.
Are you sure about that ½ 20 thread ? My MK-6 carbine and MK-7 pistol both have barrels measuring .535 " + and I have no trouble sliding a ½ 20 bolt thru the barrel nut
Standard threads for a 9mm can would be either 1/2-28 or 1/2-36 (Colt pattern). My barrel from Scott fit through the front of the receiver with just a minor opening up with a 1/2" drill. You can't tell by eye that it's been altered. It's very close to usable as-is.