It's definitely walnut. Probably black, could possibly be butternut, but there's not much butternut left anymore (at least around here) after the butternut canker disease has killed most of them. Like others said, black walnut doesn't get much heartwood until it's older. You would usually see a couple inches of heartwood in one that size, but if it's in a place where it's growing fast there won't be hardly any. Sapwood is what you want for a walnut bow anyway. It should have a distinct smell to it, too, and yellowish colored inner bark.
