Hi Justin,
There are huge differences between the two species of walnut; however, within a species, the growing conditions create a lot of variation such that you can find very dense hard black walnut blanks and English walnut blanks that are weaker and softer. Nonetheless, in general, English walnut (Juglans regia) is harder and denser than American black walnut (Juglans nigra), has warmer colors, and cuts more cleanly without as much tearing or chipping. English walnut is also called European, French, Bastogne, Circassian, Persian, and Turkish walnut all of which are just regional varieties. There is also California or Claro walnut (Juglans hindsii), the lumber from which may be J. hindsii or English walnut grafted to Claro root stock. It can be very beautiful and usually falls somewhere between English and black walnut for workability and strength. If you are making a British sporting or military gun, the most appropriate wood is English walnut. Yes, wood was imported from America, yes there was a short-lived fashion for using figured maple, yes there are rare examples of other woods but if you examined 100 original British guns from the 18th century probably >95 would be stocked in English walnut. Having written all of that, I understand completely the challenge of finding English walnut blanks long enough for full stocked fowling or military guns and their high cost. When I get to finishing this gun I will explain how to make black walnut look like English.
dave