Marc, Wood, being a natural product, is variable. All walnut is not created equal. What your gun was fitted with at the factory is pretty much the luck of the draw. Lower grade guns were fitted with lower cost wood which due to being less stable may be disproportionately susceptible to cracks and shrinking. The lignin/ cellulose ratio varies depending on what part of the log the blank originated from. Lignin is the glue which holds the cellulose fibers together. Wood under compression has far greater lignin content than wood under tension. As the tree grows the wood reacts (reaction wood) to the constantly changing stresses required to maintain the vertical orientation of the tree and it's limbs. The purpose of the wood was to hold the tree up not to be a gunstock. If there is stress in the grain of the wood it may crack if the stress is greater than the forces holding it together. The stock duplicating was also somewhat inconsistent which resulted in some having more material hogged out than others. Carefully remove the lock plates. Hold the gun up to the light. If you can see a sliver of daylight between the receiver and the recoil faces on the stock it should be glass-bedded.