On a side note, i have constructed a RUBBER stock with foam filling.
The hard foam is much lighter. The soft foam is heavier, but you can (and I have) wack people hard on the head with it, and it just wumps.
The hard foam is very sturdy. These are options for stocks, as well as a hollow cast resin with foam filling, and a solid stock.
The downside to the FOAM FILLED stocks is they will have two holes at the butt end, that the foam has to purge from. I sent one to Fettdad, he can comment more on it when he gets it. (He got hard resin with foam core) and spideyfett got rubber coated foam.