Eric, how about a tapered or flared [swamped] octagon barrels.
Do they flatten them [lock area] to be parallel to the bore?
No, not that I've ever seen, certainly not on flint-era American work. In fact, the flare at the breech is often considered a desirable characteristic because it basically 'kicks out' the tail of the lock when inlet, allowing for tapered lock panels which are wider at the rear (and thus allow for a wider wrist if necessary to the style).
Later European pieces - primarily british - cut relief into the breech to bring the lock panels back *in* at the rear, but this is largely a function of neccessity for doubles which would otherwise be over-wde at the lock tails.
There are folks here who know a whole lot more about European work than I but I have never seen a flared octagonal breech (i.e. on a swamped barrel) taken down to fit a lock on any pre early 1800s piece whether American or European.