I have two installed in 300 Winchester magnums, both pre-64 model 70 heavy barrel, marksman stocked rifles I formerly shot at Camp Perry.
I have two more installed in same rifle set up 30.06's...
I have one in a Winchester model 12, 12 gauge.
I installed them myself, per the instructions, buttstock, and at the recommended up angle, so the mercury has to climb, under recoil.
I have had this system 20 years....
It does NOT do what they advertise !
It adds weight, which helps some, and the mercury moving upwards slows recoil momentum "some", but,
if I handed you an identical 70 without the device, at least half couldn't tell it makes much difference. And, I had several people shoot them, they couldn't tell a difference at all, but, they were not very recoil sensitive.
I got run over by a car, broken neck, and the resulting deterioration that comes with age. I am recoil sensitive.
I can tell a very small difference in the 30.06. V E R Y small... not enough that I will buy any more, and on the 300's, hello APA brake... the mercury reducer is useless to me.
That's my experience.