I remember seeing years ago a guide rod that had either steel or lead bearings/shot in it. The theory was that the the balls would roll backwards upon firing then slam back forward to the front of the guide rod reducing recoil. I can't remember who made them, it was about 20 years ago.
EDIT: I think the liquid you hear is Mercury. I believe they used Mercury in it also, both to cushion the balls and also add weight. These were sealed rods, of course.
Last edited by gooffeyguy; Today at 10:22..