A buddy and I built a few last year. We used 3/4 copper tubing as the case, and filled CO2 cylinders with lead for the weight. We put small springs on each end of the weight to take the jarring out. WE soldered both ends closed after adding a small amount of transmission fluid to both help things slide and dampen the shock. They were put in rifles with hollow synthetic stocks and held in place with expanding foam insulation. My buddy sure appreciates it in his 338 RUM.
