Here's what I've done so far: Originally I just dumped a bunch of BB gun BBs and bolts and washers in there. It was awkward and rattle-y. Totally aesthetically unappealing, so I just took all that out.
Then I took a few plastic grocery bags and stuffed them down into the base of the stock with a screwdriver, nice and tight, enough to cover up the buttstock screw. Then I took a handful of 180 gr. 40 cal bullets (bullets not rounds, I figured I'd never load those because the only 40 I've got is a Glock) and dropped them in. Then I put another plastic shopping bag on top of that, jammed it down with the screwdriver. Then another handful of bullets, another bag, alternating until I put right around 100 bullets in there, then capped it off with a few more plastic bags. It's completely silent and feels more balanced. I like it.
I've got a limbsaver pad on the way, so I'm not quite done yet! Heh.