Lancaster, mercury likes aluminium. Likes it that much it dissolves it fairly quickly. I have seen the damage the mercury from a thermometer did to a passenger airliner. I can't remember if it was a 737 or DC9.

The reason mercury recoil reducers are likely to be discovered is most baggage is exrayed at some stage. Those recoil reducers that I gave the link to have a hole at both ends. now if the rifle was exrayed and customs did not believe you- though the x ray should show clearly what they are- take the butt pad off and they can see.