The air travel factor is my main concern.More about weight than anything, and as mentioned in previous post hazardous materials could be a valid concern at some point with travel. Pretty sure they can tell in X-ray the difference between a weight and mercury reducer. A good smith can add lead in places you’ll never see and achieve perfect balance which is really important in big game rifle
I use the Dead Mule brand recoil reducers. They employ a "mechanical piston", without mercury.The air travel factor is my main concern.
I use the Edwards Recoil Reducer. I have it installed in most everything from 7mm Mag up to the Kgun. Hard to explain and virtually no one I have ever met has heard of it. I met Kevin at Shot Show a few years back and have become a believer.For quick and dirty on an O/U skeet gun, I filled up the stock bolt-hole with lead shot, then added a little silicone calk and compressed egg carton on the back end. Just food for thought.
IIRC I did not like how much air space the mercury recoil reducer had. This was a long time ago, btw.
I think @Philip Glass talks about the Brako recoil reducer working well in one of his threads. But that was on a Tikka 7MM Mag.