If it's the Choate Varmint stock, the stock bolt is in the pistol grip, the stock itself is hollow as LO stated, just add the weight needed, preferable at the rear so it doesn't take as much, fill the front of the stock with packing peanuts for a temporary fix, or non-expanding foam for a more permanent fix, since the volume is very little, even a sock with lead shot may be all that's needs.
Tim