That is such an amazing build, and though this thread is old, I'm hoping that I could get some help.
Currently, I have the ready availability to pick up a UK-59 barrel for $75. It's pre-threaded for a Mosin Receiver, but will need some light gun smithing.
Alternatively, I could pick up a barreled receiver, and do this process that you have shown.
I am totally ok with either one, and I am willing to take on the cost of either project. Who knows. Maybe I'll do both.
But, my question for you is, if you have a barrel sleeve (the 12 gauge), how are you fitting on the muzzle break? Is it connected to the actual barrel?
I can't seem to wrap my head around it.
Any help would be great! Thanks!