There's probably some importation problem because most of the good low-end side-by-side receivers are coming from Turkey these days. I don't know all the ins & outs of 922(r) BS, though.
Hard to get virgin receivers for double barreled guns, ore at least ones that never had a stock. Assume a US manufacture could supply them, but limited market between being NFA and many states allowing SBS.
One of the guys did a run after John and they sold slowly and will not be doing more, or so the story goes.