colorado & BB:
Apologies for the confusion, guys. ...actually I blame the grog!

I didn't follow the maths in detail on your previous posts, but it seems you may have overlooked or forgotten about boring or reaming out the monobloc prior to fitting the new barrels. Only the barest minimum existing wall-thickness is retained at the front of the monobloc, although a little more meat is needed if they are to be threaded-in of course. At the very least, the rim-recess of the existing 12-bore chamber has to be removed. If you factor this in, you should get a little better than 3/64-inch!

Holy Cow! Can't believe the asking price of 'ordinary' Greener shotties over there! That's $3,800 in our money, and a scruffy example to boot! ...and what are those brass hardware-store screws doing through both sides of the stock head? Sheesh!

At that rate, I could turn a profit of AU$6,750 on the last two I acquired!!