Doc,

On the pad I suggest a pachmyr old English. The F990 is butt ugly and will not look good on your double.

As for mercury tubes, they work great in bolt actions and single shots. There is liquid mercury in the tube which provides an inertial counterweight to movement of the rifle rearward during recoil.

I am not sure how a merucy tube would affect a double however. Keep in mind that a double will still have one lock cocked while in recoil after the first shot. Could the inertial countereffect cause the second barrel to discharge? That's probably something to check with your gun's manufacturer before installing a mercury tube.