Sorry I forgot to discuss this.
The sand sock is used under the butt of the rifle, like a rear bag but far more versatile. It's impervious to weather, easy to make or replace and can be filled with any manner of filler materiel. Filler materiel can be, rice, sand, poly beads, styrofoam beads, beans, kapok, foam cushion, gerbils or other small rodents, gold nuggets or whatever can be shaped to fit the ground and rifle butt. Remember that it'll eventually get wet, dirty, smashed, whacked and frozen so pick something less effected by these conditions.
Here's a commercial version under the buttstock.