It's important to remember that the "growth rings can't be violated" dogma is a tradition of (civilized) Anglo-Saxon archery and that's why it's so popular on web forums such as this one. "Primitive" stone age bowyers were not aware of this "law" and thus "violated" growth rings frequently and violently. It would be impossible to build a true duplicate of an Ishi bow or a Cheyenne horsebow if you didn't violate the back of the bow, because that's exactly what they did. Honestly, after violating growth rings time and time again, and producing unbacked bows just won't break, I'm inclined to say that the primitives got it right yet again, and that this is nothing more than an Anglo-Saxon superstition.