The American walnut is a much straighter grained wood with much less pattern. In heavy kicking calibers it is usually left out since wood fibers have a tendency to split under heavy recoil.
The Turkish walnut is more randomly grained and allows a lot more recoil abuse.
Both are nice, the American is perfect for consistency(look wise), the Turkish is random: sometimes exquisite & sometimes good for the woodstove.Certain models are now available in fancy grade walnut. I bought a CZ-550 magnum which even the competitors stock does'nt rival in quality & beauty.
