A double shotgun barrel is not neccesarily straight. It needs to be adjusted a bit for both barrels to shoot to point of aim. It would be nearly impossible to do this if the holes were drilled in a piece of steel that includes all the parts you mention. Yes, it is being done, but not neccesarily successfully.
Bill is very correct. The English used a pnuematic device early on to straighten barrels once they were bored. This image is of Lucien Clement's straightening operation circa 1890 in Nessonvaux.
The original monobloc patent (1881- Henri Pieper) won all sorts of awards, as it was recognized for it's strength. Beretta still uses it for most of their guns.
Pete
