I have them in all my guns. I usually use the 11 oz version, which is 7/8 x 4". You will find that this almost perfectly fits the stock bolt hole with very little sanding. I usually place a velcro pad where it contacts the stock bolt and angle cut a piece of dowel at the pad end. Keep the dowel slightly proud of the stock so that the pad will hold the recoil reducer firmly. I carry a threaded bolt that screws into the top of the reducer, in case I need to remove it to access the stock bolt. Finally, I do not like the reducer to be at all loose, so I use clear packing tape to fine tune the fit to the stock bolt hole.
I find that it really does help with recoil...on some guns quite a bit (Model 12). However, truth be told, I really doubt the mercury does anything other than provide an elegant way to add weight. I don't believe the mercury "moving" does a thing. I think what I am really doing is just adding weight. I like a heavy trap gun.
I've played around quite a bit with barrel weight, and found that it did nothing but screw me up. I guess the whole "gun balance" thing doesn't much matter to me, but barrel weight sure does. In the end, remember that we premount and lock the gun into our shoulders. This mitigates a great deal of gun balance (ymmv).
Anyway, that's my story and I'm sticking to it....
Paul