I think trying to buy mercury for a project like this is futile. Where would you get it unless you had some stashed away.
I added a set of 32" barrels on to a hammer gun that I like to shoot. I set it up for trap shooting. I added a center bead and also .157 ivory (mammoth) front bead. I put a beavertail for-end on it, and now the balance point was barrel heavy. I remembered the stock had 3 3/4" holes in it. Two were about 6" deep and the other was less. I had some .750 aluminum and turned it down to .700. Still was barrel heavy. Drilled a 1/2" hole 3" deep and taped the opening to 14mm, perfect for the 1/2" hole. Made a cap for it and filled it with #6 shot. After a few trials it now balances 1/4" past the hinge pin.
You can see in the picture the total weight of everything. I didn't do it for recoil but I'm sure it will work. I only shoot 3/4 oz. of shot so I am not worried about recoil.
