RE: Filling a Synthetic Rifle stock?....with what?
I have the same gun. Yes, it kicks a little more than tar out of you. I havn' t been shooting it in a long while but I am going to put some store bought sand in it. Then cut a thin piece of rubber to seal around the edges and cut to fit. That should take a heck of a lot of kick out of it. That sand should absorbe a lot of the kick before it gets to the shoulder.