It doesn't look like anyone specifically addressed "mercury recoil reducers." I believe the theory behind them is that when recoil pushes the gun back, the mercury initially stays in the same place ("an object at rest remains at rest" kind of deal). A fraction of a second later, the mercury's momentum moves back in the direction of the gun, but the idea is that the recoil is being spread out of a longer period of time and multiple, lesser recoil impulses instead of one sharp one.
Perhaps the best way to visualize this is to think of a gallon jug of water, 1/4 full. If you try to slide it across a table, it won't slide in one smooth motion. It will move a few halting jerks as the liquid shoshes back an forth within the container. With the mercury recoil reducer, the gun is the container, and the mercury is the liquid. Obviously, 14 oz of mercury in a 7 lb. gun is different than 32 oz. of water in a 6 oz. jug, but the premise is the same. I suspect putting a weight between two springs in a stock is supposed to work along the same theory--objects shifting at slightly different times.
As oneounceload said, more weight can reduce free (or actual) recoil. Adding 14 oz. to a 7 lb. gun, for example, would reduce free recoil (or actual recoil) by about 11%, whether it's a $70 recoil reducer or <$1 worth of lead shot. Whether or not the the recoil reducer does much more than add weight is questioned by a lot of people.
Last edited by idek; June 7, 2011 at 05:05 PM.