I was tired of waiting for ATI to import the spare barrel and non-existent adapter.  So, I had Morgan (eastTXhunter) with Dallas Shooting Supplies cut and thread my barrel.
Here's a link to his EE post Barrel threading

Morgan cut, crowned, and threaded the barrel at 8.25" with a M9 x .75 RH thread.  He duplicated the GSG 1911 barrel threading so I could use the SilencerCo threaded adapter.
During the initial mockup, my barrel became loose in the molded pot-metal barrel extension.  This happed while I was tightening the adapter to the muzzle.  What I needed was flats on the barrel for a wrench so I could torque the adapter down on the barrel without the barrel spinning.  I also drilled and pinned the barrel to the extension.  You can see the pin on the breech end of the picture below and the added flats.


My cocking tube need modification to clear the new barrel pin.


I now needed new flats on the SilencerCo adapter, since the old flats are now recessed into the Cocking Tube:


Here is the muzzle end in various stages:




And the assembled SBR:


I'm playing with the idea of buy a GSG-5P handguard and making a Form 1 can with a larger diameter.

Nice job!  Have you had a chance to shoot it in the new configuration?

Yes - Nice work.

Pinning the barrel - Great idea!

I had a family vacation last week, so I haven't been to the range yet.  It's on the list of things to test on the next trip.

Just buy an Airsoft MP5SD and use the airsoft "can" as a shell for what you already have there...