Reading Walnut

Gunstock blanks have what I would term "features of perfection".  Those features have to do with aesthetic issues and strength issues.  Few gunstock blanks that come out of a walnut mill fit all the criteria of perfection, and those are very expensive. When I go to the walnut mills around me I sort through what may be a hundred blanks and I might find two or three that have all the right things we want for an airgun but maybe lack a feature of perfection that doesn't affect us yet keeps the price down. Understanding the features gunstock wood demonstrates will better help you recognize the value of a piece you might be looking at.  I've seen plenty of nice walnut gunstocks on airguns split across the wrist because the stock was laid out upside down on the blank or the blank was not suitable to begin with.  After reading this information, you'll be much better informed to evaluate guns and wood you may be looking to buy.
Features of strength has to do with the strength of the wood itself, and the direction of the grain flow.  Typically we look for wood that has the grain flow up through the pistol grip and wrist and also sweeps slightly up through the forearm.  Straight grain blanks are the least expensive, but gunstocks made from straight grain wood can shear at the wrist.  If you look at the wood grain going straight across the thinnest part of the grip area of a gunstock and it goes basically straight across, you can see how the wood can break.  If the grain flows up through the pistol grip, it is very strong.
Sap wood can impart an interesting look on our airguns that don't require the strength of a .300 Win. Mag., but it is important to understand that sap wood in the blank is generally considered a price detractor.  For airguns, a little bit of sap wood is just fine as it can impart an interesting look (and few people will cotton on to the fact that the colors are sap wood).  Some sap wood in a blank can help a piece that might have great figure or color be attractively priced.  That said however, I don't accept just any blank that has sap wood.  To be willing to accept it in a blank, the blank should have some other very attractive features, and the sap has to be in a position that it can be utiliized as a feature.  Sometimes I spot blanks with sap wood but the sap wood will be outside of the gunstock.  Such a piece can help keep the price down.
Another component of strength is potential for warpage during carving caused by grain runout.  Grain runout is another consideration that has pluses and minuses. By runout I mean if you look down at the top and bottom of the blank rather than the side, does the grain flow straight down the blank, or does it curve out the side (runout).  A blank with grain going straight will obviously not be prone to changing it's shape during carving, but is also rather rare.  Virtually every blank will have some small level of runout, but excessive runout can be a problem, yet can also create stunning patterns. Runout means the stock should be carved 1/2 way and let it finish moving, if it does move, before finishing the carving.  Such a blank will finish straight, and runout can create very interesting patterns on the side. Reading runout in a blank is very challenging because it is the roughest cut on a blank, but when identified as such, either through recognition of the pattern on the side or by being able to visually see the runout through the rough cut on the top and bottom, can keep the price down on an otherwise great piece of wood.  But some blanks may be made more spectacular by the feature, so there's not necessarily a price reduction, maybe the opposite.  An educated carver will recognise blanks with this feature and treat them accordingly.
Aesthetic features have to do with the flaws of nature and the magical appearance of the wood.  Traditionally a consideration of value deduction is small knots from tiny branches, fissures, and voids, and while a 'perfect' blank with high figure, perfect grain structure, and no tiny knots or holes is the holy grail, they are also expensive. Holes or little 'birds eyes' twigs don't bother me. If a stock meets all the other requirements and is beautiful, but comes in at an attractive price point because of small knots in the figure, that's perfect for us airgunners. I epoxy fill those voids and they completely disappear.  I look for details like that when sorting wood, and as with sap wood, a blank with knots in it had better have some very redeaming qualities, but often when the wood does have those other qualities, they so completely drown out the odd little birdseyes in the figure that the eye doesn't notice.  However, such a piece can have an attractive price point for an otherwise spectacular blank.
Figure is of course a prime consideration. Northern California Black Walnut (Claro) which is the wood I'm surround by, is world renown for it's gem like quality appearance. The 3d rippling wrinkled wood look, the reds, yellows, greens, and browns of contrasting color, and the potential for dark swirling mineral streaks almost as dramatic as English make for heirloom quality gunstocks. It is a challenge to see figure and color in a blank in it's rough state. Figure that will amaze you and 'pop' when the grain is filled with oil and polished is virtually invisible in it's rough hewn state at the mill. This is where the experience of reading wood comes in the most, especially when sorting junk after junk at fairly high speed but spot the gem through a rought bandsaw cut in a moment. 
Considerations of figure include: there might be significant figure on one side of the blank, but none on the other. That means the figure is falling off as it passes width-wise through the blank. Will there be any left on the good side after the stock is carved down inside the blank? Is that figure showing on the near side or off side? Blanks with figure on only one side are much less expensive than a blank that is fully figured from one end to the other equally on both sides. More than that, a blank with proper grain flow AND fully figured become quite expensive. and few airgunners are willing to pay the price of a Daystate for a chunk of wood. 
Ultimately what I look for is wood that meets all the requirements that are important to us airgunners but misses some aspect that keeps it from being thousand dollar wood. I particularly look for wood that was mis-graded by the grader and has good contrasting color, dark mineral streaks, or interesting figure. If you've kept an eye on the wood and prices I pay for it and compare to what you can buy online direct from mills or on eBay, you'll see I get some really spectacular wood at great prices. But the primary way I accomplish this is finding the hidden gems in the stacks that were mis-graded by the wood grader. I have long experience reading walnut. I've been doing this since I was a teenager (I'm going to be argh 55 in a couple months), so I can spot the good stuff in the rough cut and dirty state walnut blanks are typically in at the mill when I go searching. So most of the wood I get at attractive price points is more because the wood grader mis-read the wood. This is how I am able to provide wood at a grade or two or three better for the price. I feel this is the biggest value I can bring my fellow airgunners, allowing them to have heirloom quality stocks at our kind of price point. This is in fact for me the fun I have going to the mills. One, it's a journey. Some of the mills I go to are three hours or more away, and then when I get there it's an adventure, sorting through a ton of crap wood looking for the hidden gems. And then when I find it not to act excited when paying! That bit me once.