http://www.brownells.com/aspx/NS/store/ ... SUPPRESSOR
Take a look at the description for one of the many that are on the market, I happen to have the one that fits in the barrel and I place it in the top barrel when shooting singles trap in my Fabarm O/U.
Mercury is used because it is a dense metal that is a liquid at room temps. I'm sure you could concoct a sand and oil mixture or plastic beads and oil mixture that would act similar, but mercury is readily available and not likely to be as temp dependent as some of the other possible creations.
The containers that it comes in are well sealed, I'm sure if you wanted to you could hacksaw into them, but I've dropped mine a time or two and no silver liquid has come out.
You can get a similar effect from just adding weight, but that is just working on the principal that it requires more energy to move more mass, so there is less felt recoil. More energy is used with the sloshing of the mercury.
Another option is something like a Deadmule
http://www.brownells.com/aspx/NS/store/ ... IL+REDUCER this contraption is a spring that counteracts the recoil force, similar to a shock absorber. Also something I happen to have, this in another gun that was heavy enough to start with, but as I have an old shoulder injury from stopping a 12ga slug I try and limit recoil when I can, most of my long guns have at the bare minimum aftermarket recoil pads.
As to were you want to put the weight, well that is more about personal preference, there are mag caps that have extra weight to them, there are barrel mounted rigs that allow you to position the weight were it best serves you far forward for barrel heavy or father back for a lighter feel, you can put them in the magazine, the stock, in one of the barrels....its where it best balances out the gun for you.
Look at some of the new bows on the market with the internal shock dampeners, small brass disks held in place by some rubber, the vibration is spent shaking the brass disks around.
There is all sorts of physics involved when shooting...more than I can remember from either high school or college physics.