filling a rifle stock...
 

what is the best thing to fill a rifle stock with for added weight?

Lead shot or ingots are the stand by. Mercury recoil reducer would be another good option. I did a few several years ago by melting lead shot into a long skinny ingot, routed a groove in the belly of the forend, set it in place and secured it into the stock with accraglass. Simple job.

Adding to the buttstock is simple as removing the buttplate, drilling a hole, and placing the shot or ingots in the stock. I'd only do that if I had too much weight in the muzzle and the rifle didn't feel right, but it'd have to be a DANG heavy muzzle (I prefer heavy muzzles). Again, secure the shot or ingots inside the hole by sealing behind it with bedding compound or wood glue, even hot glue if nothing else.