Yes, more mass in the gun lessens felt recoil.
Lead is cheaper than murcury recoil reducers, but they both do the same thing. If you use lead shot instead of cast lead, make sure you pack some wadding in tight behind the shot so it doesn't shift under recoil (negating some of its mass).
BTW, the mossburgs (and several other pump shotties) have aluminum receivers to make them lighter in weight.
Makes sense. Thanks.