This is NOT the case in most instances in the US, or when traveling under FAA regulations and guidelines.
The FAA states that consumer products that contain mercury are absolutely allowable.
They go on to further state that medical devices and other non consumer devices such as commercial grade weather barometers that contain considerable amounts of mercury are also allowed, but must be declared by the passenger and that they are only allowed for government officials, physicians, etc..
In the case of commercial grade items and medical devices they can only be located in carry on baggage and must be inside a leak proof, puncture resistant container..
Neither the FAA or TSA have any policy, nor is there any law, that prohibits traveling with a mercury recoil reducer that I can find after searching FAA regs, TSA regs, both agencies websites, Lexus-Nexus, and doing a basic google search..
If you have information to the contrary, I'd like a link (sincerely).. I absolutely intend on traveling with a rifle that has a mercury recoil reducer installed in it this summer..