from The Modern Shooter by Captain Lacy published 1842:
The neatest and best form of barrel for all single guns is
what is called " a Nock's form" barrel, or one which is flat
both at the sides and top of the breech, instead of being circular,
like the rest of the barrel.*
Let your duck-gun stocks be made of good, well-seasoned,
walnut wood, the less veined the better, for figure is no object
here ; and, as I have before observed, the plainest are generally
the strongest stocks, and are, therefore, the best adapted
* You cannoi well keep a round barrel in a vice, and no clams will hold
properly to turn the breech out. A single barrel, especially a large one,
should always be "Nock's form." Moreover, a round barrel does not make
near so good a finish in workmanship.