Go to the home page for this BBS, and click on the advertisers drop-down list, click on Pete Hiatt Gunstocks, and you will find a lengthy discussion of all kinds of information about walnut and other gunstock woods, including some pictures of stock blanks containing graft wood.
From my limited experience, two precautions; 1) the light graft wood will not darken, and 2) the 20 inch diameter is about the minimum size to get a useable gunstock blank from, really borderline. The reason for this is that the stock blank cannot contain the dead center of the heartwood, which will produce shakes and checks in drying (splits). This is what wood people call "boxed heart," and you have to cut the blank so as to miss this area, meaning you only have half a log to work with, which may be only 10 inches or less, after you cut out the boxed heart, the bark and the sap wood.
Bill
