I was thinking the same thing, you could adjust the amount of shot to balance the rifle to your liking.If you really want to balance it out you might be best served trying to fill the empty space in your stock with a pouch full of lead shot or something.
A friend of mine shot with the Marine Corps Rifle Team for a couple years and his competition M16 was something like 26 pounds with all the lead in it. Basically had zero recoil firing the puny 5.56 round even in rapid fire.I took all the plastic off my PTR91 and put on wood. I did not notice a change in weight or distribution.
I shoot an AR with a 20" heavy barrel a lot in Highpower matches. For awhile I put more and more Lead in the butt and under the hand guard. It got over 15 pounds. Recently, sanity returned and I pulled it all out. Vast improvement. I shoot it as well as I ever did.