Stock Repair
 

As I understand it the lead will build up a crusty layer of oxide from it's environment. This is how lead corrodes or oxidizes and this build-up is what will split the wood...

I don't believe it was poured into the cavity in a molten state or it would have charred the wood and eventually you would have a heavy lead slug sliding back and forth inside the stock. I believe the lead slug was tapped into the bored recess and waxed or glued into place.
The oxidation would have occurred from a damp or moist environment over time.





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