With ash the seasonal growth rings run alternately, and noticeably, harder and softer, the spring growth being softer than the dense summer growth. In other words the softer part is rather punky while the hard stuff is like iron. This can be a problem when working it with hand tools. For instance when hand chiseling off some aspect of the build you will naturally be applying force to cut the dense part and when the chisel gets through that you'll find that it wants to suddenly zip through the soft section and on into something you didn't intend to cut. With ash chisel cuts are more difficult to control than with hard maple or even cherry.
I have stocked a gun in ash, I would do it again. Ash makes a beautiful longrifle stock. Knowing what I do about ash would I recommend it for a first build - no. For a first build I would recommend hard sugar maple - no question about that, you would be better off learning on a nice even grained hard maple stock.
dave