Bob:
The .577 barrels are sleeved into the 12-bore "monobloc" in the normal way. The cut to remove the original barrels is a little further forward than usual for a shotgun re-sleeve, and you can see the joins in a couple of the photos above (although they were quite well done and are certainly not obvious). The breech-ends of the sleeves are visible in the "greatest assets" photo above.

My regulating load is 77gr of ADI's AR2206. This gives ~1665fps for the 650gr Woodleigh Weldcore. I drop the charge by 2gr for the steel-jacketed solid, just to be on the safe side.