There's probably a variety of different structural foams used by 'glas stockmakers, though the only makers I have personal experience with are McMillan & Robertson Composites. From doing some inletting on both brands, whatever they're using appears very similar, and is very dense, unlike what I'd expect to see with any of the expanding sealing foams sold in lumber stores. None of the stocks I've worked with were intended for weight-specific BR class shooting, so perhaps they were built using a heavier, higher-density type foam. I know that when you want a heavier stock from McMillan, you can specify solid fill for the butt section, but am not sure whether they use a different foam filler, or just don't leave a hollow space in the buttstock.
