Ironwood,
A swamped barrel is the same inside as a straight one. given the twist rate and caliber, and rifling cutting technique.
Yet some or most of the outside octagon has been removed from the center in an even and balanced fashion, thus lending a lighter barrel and changing the fit and feel of a barrel in a given stock/frame.
The added work to "swamp" a barrel doubles if not more, the cost to own one.
Most of the best "off hand" shooters use a swamped barrel,due to it's lightness and superb balance.
That's just a simple explaination,Make sence??