It all depends on how the Walnut was cut and cured. Cross cut of the grain, parrallel with the Grain, Vertical with the Grain, is it clean clear of knots, or does it have a lot of cracks and checks from drying to quickly? all these need to be considered before you can say yes or no if the Walnut will make a good Bow.
Is it all heart wood? Combination of Heart wood and Sap wood? Did the Walnut trees come from bottom moist land along a creek or River where it would grow really fast, or was it grown on high ground, all these factors come into play to determine what quality of Bow you will end up with.
But YES Walnut does make a very beautiful Bow if it is cut Flat Grain, fast, and hard hitting Bow, if it is backed with F/G On back and belly. In fact it is one of the better Wood laminations for making Bows....ONE SHOT...