Tiger stripe, fiddleback, curl, whatever name it goes by is caused by food material (photosyntate) produced by the leaves and sent down the trunk to the roots for storage. As trees age the roots tend to fill up with stored food material, and the excess produced is then transported to the center of the tree by the storied ray cells. It is this food material in the storied ray cell that produces the off-color curl or stripe. the food material is nothing more than one of the 8 or 9 basic sugars of which our common table sugar is one. If you look at wood under a microscope you'll see these ray cells running at a 90 degree angle from the rest of the wood cells. It can be very evident in the white oak group in quarter sawn lumber as was pointed out about Victorian furniture. A number of species will show stripe/curl, whatever, but it seems to be more common in Maple, Ash and Walnut. It does not weaken the grain structure of a stock blank in any way.
Curley Maple, both hard Sugar and softer Red maple were the woods of choice for 18th century flintlock longrifles made in Pennsylvania and other areas. Occasionally a Walnut stock shows up, but usually it was Maple with curl/fiddleback/stripe. Most New England, Hudson River and British Style Fowlers were stocked with American Black Cherry, and it is unusual to see any curl in these.
While I'm not familiar with selecting stock blanks for modern doubleguns, I am up to speed on stock blanks for contemporary flintlocks and caplocks. Curl increases the value of the blank, and the more (tighter is the trade term) curl it has the more valuable. Five or more stripes per inch significantly increases the value. A friend who is in the sawmill business will use a hatchet and closely examine likely trees for curl, and when found will usually dig down below the root collar and cut the tree off maybe a couple of feet below the root collar. When sawn, this will produce a downward curve of the grain pattern through the wrist of the rifle or smoothbore fowler or whatever is being built.
It's a shame that I'm not smart enough to figure out how to post photos on the Board. I could show a few pieces with various amounts of curl/stripe and desirable grain pattern. I have a flintlock doublegun, 16ga, stocked in English Walnut that has beautiful stripe and grain patterns that would illustrate what I am saying very well. Here is a website of a friend who has built several pieces for me including the doublegun. this will give you a good illustration of several patterns of stripe/curl.
http://www.dtdodds.com
