Other species of American Walnut trees are indeed used to make gunstocks. Eastern Black Walnut is the most commonly used because it is the most widely grown species (in North America). But even white walnut (Butternut) is suitable for gunstocks. Ever heard of Claro (California Black Walnut, i.e., juglans hindsii) being used for gunstocks?
Here's a reference to some, but not all of the species of walnut native to North America:
http://www.mast-producing-trees.org/2009/11/native-walnuts-of-north-america/ 
