Dr. Tim,
You may rest assured that vast amounts of American Black Walnut (ABW) (Juglans nigra) was shipped to Europe from the Colonial Period through the early 1930's for gunstock material. This is well documented and anyone knowledgable of researching on the Web will find this. The Germans, especially, liked to stock with American Black Walnut finding it superior to European Walnut (Juglans regia). Virtually all German military weapons-shotguns and rifles-in the WWI era were stocked with ABW. And, German WWII K98k's up until about 1935 or so were stocked with it also. It was the loss of ABW on the eve of WWII that caused the German arms industry to start with laminated wood for stocks. Then late in the War, late 1944ish, you suddenly find some 98k's again stocked with ABW that had been hoarded back primarily by Mauser for top-end pieces. Roughly made and thrown togather 98k's with beautiful ABW stocks! Take a look at high-end Deutsch pieces, Oberndorf Mausers, J.P.Sauer und Sohn and other upper end manufacturers of the late 1800's-early 1900's and you're much more likely to see ABW than European.
There are a number of factors that will influence/govern the quality of any one specific species of hardwood, and none of these have been mentioned to any degree. ABW, for example. Here where I am located is just about the most Southern tip of its natural range. I have logged a lot of it out over the years, and all of my kitchen cabinets as well as a room and a lot of trim are panaled with ABW that I logged. Beautiful to look at, but I wouldn't give you .15c for a gunstock made out of it. But....I have a couple of rifles that Dave Dodds made for me that are stocked out of ABW from the Upper Pen. of Michigan, hand selected by Wayne Dunlap, and you will not find a more superior piece of wood for a stock. Where a tree grows will have a big influence on the wood quality. European Walnut grows from England across Southern Scandanavia to central Russia and south through the Medeterrian, Greece, etc, to Spain and Portugal. The quality in one area will not be the same as in another area of its natural range.
Some years ago I was selected/scheduled to go into an area of Africa to "build a sawmill and cut/ship lumber to Europe". I studied about 15-18 different species of tropical hardwoods that I was to saw, and I can tell you that 3-4 of these would make gunstocks superior to most woods that we use. I don't know why these aren't selected and used now for stocks. Maybe these species just aren't available on the market.