Looks really good.
I like the shoe lump option especially for a second set of barrels and seems like it is much more straight forward than a threaded mono-block. No seam to conceal, different metals and blue characteristics,etc.
One thought though, in looking at the breech end and knowing that there will be a quarter rib fit, is anyone making concealed longitudinal cuts along the barrels in an effort to shed a few ounces?