I have heard of people doing this with that filler foam. In my CVA Mountain Stalker, it was such a light weight rifle, I put bird shot into the stock and stuffed it with old rags. I put the bird shot in a vacuum sealer bag (twice), put a old clean rag in first, form fitted the bird shot to the stock, and then packed it with more rags. It not only made the rifle quiet, but put a little extra weight in the stock and made it a little more comfortable to shoot.