Those lines are called "Fiddleback" and in most cases they are a character of the wood and are desirable.
Gunstocks are traditionally cut by a method called "quarter sawn". The log is quartered laterally and then each quarter is then cut perpendicular to the grain. That way each cut produces a segment with grain showing (somewhat difficult for me to describe). The fiddleback is little involutions and undulations in the grain.
The value of the blank has a lot to do with its location in the quartersawing process and of course the individual character of each piece. Quartersawing produces a lot of waste but also produces the finest gunstocks.
All things are a product of their environment, so, Yes, all walnuts are not the same and do vary in certain characteristics by specie and location grown. I am far from any sort of expert on this subject. I am only repeating what I have learned by listening to others.
I have done some milling (I have a Timber King saw mill). I have never quarter sawn a log, but I suppose I should sometime.
Alan
