I actually have two mercury recoil reducers in a Winchester 1300 pump gun that I sometimes use for trap. One is located in the buttstock, and the other is in the magazine where the plug was before. The added weight does not bother me for use as a trap gun and helps smooth out my swing. The felt recoil is less than before I added them, however it is not less than my gas operated 1100's.
As to whether they work or not? hhmmm..I am not sure whether the mercury really helps, or if I would have been just as well off with simply adding the weights to the gun. I might just add the weight if I had it to do over and see how it worked for me.
Good Luck!