i would think for the sake of this question that although walnut is more complicated than just english or american black, these two subspecies would be the main focus, especially when the question is dierected towards gunstock wood. it would either be wood from the locale or wood imported from england, not to say that that's the only place "english" walnut can come from today, but the most likely at that time.
english is definately better in terms of the requirements for carving and intricate shaping, as stated, it is denser and it's fibers are more closely intertwined, which makes it stay together better under the gouge or knife. i have af air amount of experience with american black in the furniture and clocks i build and i have not seen anywhere near the shapeability of it compared to the few pieces of english i've had. it is generally more brittle and will flake off in small seperated segments as you cut a radius, where english will more generally release a segmented curl as the gouge is advanced.
now, allot of that may be the result of how it was kined and cured. the large majority of today's black will most likey be kilned for furniture building and production stock making where machinery will actually do a better/cleaner job of shaping, while english is almost exclusively cured with gun stocks in mind. in the time that is of intrest in this thread, both were more commonly used in stockmaking(i would think) ans there would be allot more present that was cured with that in mind. kilning any wood will produce a more brittle product and when you through today's "bottom line" economics into the mix ,allot of very good lumber ends up one step above good expensive firewood.