Polyurethane foam. "Great Stuff" (as mentioned by a previous poster) is one brand. Works perfectly. Just tape off the outside of the stock, to prevent any spill over from bonding to the outside of the stock. The poly foam is VERY sticky, so it will permanently bond to the inside of the stock - and won't ever get loose. It will automatically fill all the spaces and voids inside, so you don't have to pack it in, as you would something like Bondo - so it's much easier. The foam will deaden the hollow sound of the stock nicely. It will also make the butt area of the stock much more rigid.
The poly foam is light weight, so it adds little to the stock by itself. You can add additional weight simply by pushing some other material into the foam, such as shot, metal washers.....almost anything. You can easily balance the added weight by not adding it to the rear portion of the butt stock, but rather, further forward. Just apply the foam in "lifts", or layers. In other words, add a small amount of foam into the forward most part of the cavity, then add some weight....then add more foam, etc. etc. It's quite easy.
After the foam is set, simply trim any overflow with a razor blade...and re-install the butt cap or pad. Voila.
I've done a bunch of stocks this way, over the years.
Last edited by wpsdlrg; November 24, 2012 at 08:24 AM.